The simple way to decide is if slotting end reds will reduce your effectiveness more than giving up 3 power choices for stamina.
In my build as a Bots/FF MM I can slot end reds in the powers that need them with out losing any effectiveness from the power. If I had taken stamina I would have spare slots which might be useful but I would alos have lost 2 useful power choices (would be 3 apart from the fact that I cant decide on what power to take at lvl 35).
In my build stamina would only be an asset when using force bubble as usually I dont need to resummon during fights so my normal end cost is low. So for me stamina is not a good thing as I can quite happily run 3 toggles (Assault, Tactics, Dispersion Bubble) as well as heal (Repair, Aid other and Aid self), attack (Force Bolt, Repulsion Bomb) and rez (Resucitate) as needed duirng a battle. For me my one end hungry power is more of a situational one than anything else due to the aggro generated and the end cost.
So for me the choice was that I am better off with my power choices and using end reds than giving up powers for stamina.
Just to add another viewpoint 
Personally I love stamina on my Robots/FF as I was without it for 37 levels and respecced into stamina at 38. I would not look back after playing with stam vs without it.
Ok I know Master Zap isn't a fan and neither are others, but I personally love it!
It's all about playstyle in the end and how you want to play your MM. My reasons to take stamina are:
1) I don't really have any other 3 powers in the FF set I particularly want to replace stamina!
2) Running 4 toggles and Force Bolting / Resummoning mid fight in a tough team absolutely kills your endurance. Especially putting 2 force fields on a team of 7 other people with 4 toggles up mid fight. Without stamina that was my entire endurance bar gone ;P Also just bubbling your bots before a tough fight took tons of endurance with 4 toggles up, and to be honest I didn't want to turn them all off just to rebubble!
3) Being able to summon and buff everyone at the start of a mission without a few blue insps was a joy!
I have also got my Necro/Dark to lvl 29 at last and I have to say it seems to be a lot easier on the endurance than my robot/ff is, however I built in stamina from the start. The problem I am having is /Dark has soo many great powers, I want the 3 power slots now!
Any /Darks out there with their thoughts to no stamina, and taking 1 or 2 personal attacks?
Do you run out of endurance really quick, healing, debuffing and attacking (I would be running 3 toggles + darkest night and attacking with no stamina!)
The reason I ask is for i7 and the Thugs set...I want dual pistols for concept reasons!
- Kal
merc traps here and have been staminaless for all of my 36 levels.
I have to rest occasionally, and I'm constantly having to turn off leadership in long fights but I have many more traps at my disposal than if I went with stamina.
If the patron pools don't impress I might get stamina from 40-50 (or respec into it)
Necro/Dark here without stamina.
I really don't feel I need it either. I have one personal attack in Gloom, but I have every Dark power so far (I'm lvl 28). They're all just so good! My only pool is teleportation.. Recall, tp foe and of course teleport. The only time I feel a little drained is after summoning/buffing, but theres usually no aggro at that point so I rest up.
I have end redux in TP (1), Twilight Grasp (1), Darkest Night (2), Shadow Fall (1) and Enchant Undead (1). I only use the blast now and again so I don't feel it needs any end redux, I tend to use TP foe to pull.
Anyway, rambling now... But Yeah, Necro/Dark + no stamina = no probs so far. 
It could be just me but I dont really have end problems with summoning.
Usually I do the following:
1. Summon Assault Bot followed by Battle Drones
2. Use the upgrade buff followed by the equip buff on the assault bot
3. Summon Prot bots while equip is recharging
4. Equip everything not equiped working down battle drones to protector bots
5. Buff a prot bot with upgrade as it has now recharged
6. Activate Dispersion Bubble, Tactics and assault
7. Head towards first enemy but before engaging buff the other prot bot with upgrade.
8. Buff battle drones as upgrade recharges also shield all bots as required
I have 1 end red in equip, 2 in despersion, 1 in deflection shield, 1 in insulation shield, 1 in tactics and one in assault. I dont have sprint running and I very rarely get the out of power message. Out of power only comes up when using force bolt excessively, spam healing bots or team mates or using force bubble.
I never use blues normally only if they are in my tray and only then when facing ambush after ambush with no break in between.
My travel power of choice is teleport and I can teleport from thorn isle to the ferry in nerva without running out of end even with PFF and dispersion bubble running and possibly sprint. I do have 3 end reds in teleport but then that is only 2 extra slots so not really a problem.
I've been looking at maybe working to get stamina (am a mercs/evilfish MM) , as I like to be pretty active in missions and do find myself getting low on end after a couple of large groups (also in pvp the end gets sucked away very quickly when fighting heroes).
In the end I guess it all comes down to how you play your MM.
i respected my MM yesterday (robot/poison) into a build with stamina. Having leadership running, shooting debuffs nonstop and have to heal nonstop (with hasten/1 recharge its up in no time) i was loosing endurance big time.
Did a bunch missions, feels way better 
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
Sign the petition, dont let CoH go down! SIGN!
Bots/Dark
Have Tactics and Assault.
Went without stamina for 28+ levels, but in long fights found that while my pets were fine I ran out of end so fast I ended up losing Shadow Fall. Now I've got Stamina I can last even an extended fight with no real issue. It also helps when something tries to drain my END.
I have a level 12 alt, Bots/Traps and he won't be taking stamina due to far fewer powers being essential from Traps.
And I suspect when i create my Bots/FF they will take stamina.
Bots/Traps here, and never really needed Stamina - or endreds, for that matter, except on toggly powers (Tactics, Stealth, Fly. Couple of EndRdx in the hungrier travel powers will cut your setup time in half.)
Even with Assault, Tactics & Stealth running I've still got plenty of power for Repair, traps and the odd blast. Let the pets take the strain...
Is it time for the dance of joy yet?
Robotics/FF
Never needed stamina throughout 40 level PvE experience. I have Dispersion Bubble, Assault & Tactics toggles on all the time, however, I intend to add Combat Jumping & Acrobatics toggles in order to survive Knock Backs in PvP. I have Flight pool atm...
I don't know whether I'll need stamina to be able to keep 5 toggles on at all times, or adding End Reduc SOs will simply be enough...
I'll test it in the training room once I7 is available because new epics pools will also be influencing my build after lvl40...
@blastor
The Intrepids SG & The Institution VG
Retribution SG
necro/dark
the only time i need stamina is when summoning pets my build has no shadow fall as it uses too much end in my stamina-less build, and im running 3-4 toggles in a fight, darkest night, tactics, assault, and combat/superjump most of which have 2 end reductions in, i have no personal attacks, and i have pahse shift and invis, just incase, grant invis is for pvp purposes, and tp foe, again for pvp purposes, but i manage without stamina, and wouldnt think about bringing it into my build
I have a ninja/dark MM with assault and tactics, and no fitness pool. I don't have any end problems running debuffs/heals/toggles during battles, and I have all the training and toggles slotted with end reductions. I'd rather sacrifice a few slots than three powers whenever possible.
Robotics/Dark
I have Shadow Fall, Assault & Tactics, Combat Jumping & Acrobatics toggles on most of the time and Darkest night on until my anchor gets killed. All except CJ & Acro are 3 slotted with end-red, which is just about doable while soloing. I do however run mainly with a kinetics corruptor which makes life dreamy, without this I would not be spamming the heal half as much.
@Terrahawk and @Terrahawk2
Octavian Vanguard
Since I dont have a respec yet I cant test whether its better to have Stamina or just slot End Redux in my most used powers. I'd like to leave out Stamina as there are plenty of other powers I'd like. Being A Necro/Poison I have to use the heal from my secondary a lot, plus of course all the debuffs I have. I thought of also putting an End Redux in the upgrading powers for my henchmen as well. Any thoughts on this?