I have a ninja/poison, it works pretty nice. Since most damage they do is single target, you can ditch out alot single target debuffs.
Slotting the heal might be usefull, since they are def based and you only have 1 -acc power. The aoe slow power works great to let mobs hit your pets less. The final pet of ninja stacks greatly with your hold power, good for bosses. 2nd tier pets with your smokebomb can nearly be perma-hide, doing nonstop criticals.
Mercs can be a choice too, but they are very very DoT-based and this can get quite annoying. Although they are full ranged, some pets tend to run in melee range for no single reason.
My experience with Mercs is very bad, even with alot of -resist debuff they still do (specialy compared to ninja) very low damage.
My ninja/poison is lvl20, quite enjoyable to play (specialy if u go 3rd and 5th difficulty, little group spawn so you can focus alot on single target). Once they all fire their attacks on a debuffs mob, it goes down fast 
perhaps just try them both for a few lvls, see how they handle and if you can enjoy the sets.
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Cheers for the input....think i might try Ninjas to start with because after all, they're NINJAS!! not only that i kinda like the idea of big numbers instead of DoT which i already see lots of on both my current MM and Corr.
NINJAS!
Starting to think about rolling a second MM was was wondering what goes well with /poison?
I'm choosing Poison because i personally have only ever seen 1 other poison MM....and the fact that /Darks seem to be everywhere, also i get slightly bored with /FF at times.
The pets would either be Thugs, Mercs or Ninjas...already got Bots and didn't take to Necro, what i'd like to know what of the 3 stated would suit poison better, already seen Thugs in action at a high lvl and i liked...but again they seem to be everywhere, so would like to here more on Mercs/Ninjas to try and sway my vote.
Will add this is a PvE only situation and Solo/Duo...although i don't know what i'm duo'in with till the missus makes up her mind