They're not for everyone. I personally don't enjoy them that much either. I don't think they're supposed to be super fun or anything, just more of a challenge.
Like any AT, they play like their own AT.
I quite like my PB, he's my main main now (I have three "main"s). Personally, I don't get on with playing scrappers, while others hate playing defenders.
The epics are ATs unto their own, some people will like them, some won't.
First of all what level have you got up to?
I find the Khel' AT's are slow to start but pick up pace and eventually bloom in the higher levels. Not that I have got that far yet. But thats what I am inclined to belive and have been told.
K
You have your ups and downs with them, came close to deleting mine a few times, when i got into the 30's...
PB's become awesome once you get their Photon Seekers and Dawn Strike.
Even at lvl 50 you still feel vunerable
Got my PB up to lvl 26 before i stoped playing it, and from about lvl 8 to 26 felt like a real grind, mainly due to the void guys i guess, they really slowed down the fights when one reard his ugly head.
Well... I've gotten my PB to level 42 so far, as she's as much fun to play as when she was level 1. The game became FAR too easy on my energy blaster after a while, and with my PB, I actually have to be far more carefull and mindfull of tactics!!
There's only one little niggle I have, and that's that I have too many powers, and not enough slots! 
@FloatingFatMan
Do not go gentle into that good night.
Rage, rage against the dying of the light.
I found that also.
Had myself totaly lost to where i should stick anything, more so then when i first started to play CoH.
BUT thank god for the guides on the forums, it did help me alot with sloting up to lvl 26
I'm happy to hear that peacebringers will bloom in the late game because I've been feeling pretty über since lvl 6 and I was worried it would inevitably get worse. 
Having 4 ranged attacks, 2 of which are fast-recharging AoEs, and unsuppressed flight at level 6 is just insanely good at that point. And while blasters do catch on, I still feel extremely strong in the AoE department at level 25. It helps that I've been teaming a lot with an ice tanker, possibly the ideal partner for a nova specced Kheld. He gathers the crowd in a tight cluster and I bombard it to death from the skies, often not getting a scratch in the process. But even without that, the Dwarf gives me versatility that helps a lot in diverse team situations.
Soloing is a challenge because of the voids and quantums, though. A minion will kill me if I'm not prepared. I hope that slotting the big melee attacks in the human form up a bit will help with that as I've now got Nova and Dwarf pretty much where I want them to be.
Having taken a big CoV shaped break from my PB for a while, and then coming back to him in the last couple of weeks, I have to say that I had forgotten how truly fun he is.
I'm tri-form all the way, and the joy of being on a large team, launching a couple of AoEs from Nova, dropping into Dwarf to take the return fire, then popping into human to throw out some melee damage never gets old.
Dead Calm's Defender Manifesto
My PB is now lvl24 and I enjoy her almost as much as I do my controler. I admit this is because it's the same RP character, but still.
I teamed a lot with PBs on my way to 50 (Hi FFM) so I learned a bit about them that way... never really knew voids were THAT much of a pain solo... when held they're fairly harmless 
@ShadowGhost & @Ghostie
The Grav Mistress, Mistress of Gravity
If you have nothing useful to say, you have two choices: Say something useless or stay quiet.
I dropped my Kheld for a LONG while (made another 50, a couple of 40s and a couple of villains in their 20s) and came back after a while, I found that mine is fun, they're not uber in anything they'll do(a tanker can tank better, a scrapper or blaster can make FAR more damage etc)but they have the versatility.
The squid form has a nice damage output, the dwarf form is really tough and the human form have some nice tricks, anyway if you're goin full human from the start it can be a bit depressing because any void/quant will easily get you in no time if you're not REALLY prepared and careful, while in dwarf form they're easy to kill so my tip would be to start as 2-3 form and respec into human only later on when you'll have more tricks at your disposal.
It also depends a lot on wich one you're playin, a PB in the lower levels is a LOT easier to play than a WS (2 self heals, BU, 2 really damaging attacks)but a WS in the late game is way more destructive than a PB(Sunless/Dwarf Mire+Eclipse+Quasar or Nova Form).
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The squid form has a nice damage output, the dwarf form is really tough and the human form have some nice tricks
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Yup, I have found Light Form is great. HUGE damage resistance and can use all the human form powers. Fly into a mob of purples in light form, unleash Dawn Strike, Photon Seekers, let rip with Radiant and Incandescent Strike, Gleaming blast and Solar Flare, the result, total annihilation and not a scratch on you.
I havent really found any love with my kheldians yet I just see them as jack of all trades, master of none AT's. Possibly there simply for the level 50 havers to be able to use their experience to partially fill any hole in a team when perhaps teaming with less experienced people.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
My PBs 14th lvl now and I'm enjoying her, but can see why some people consider them boring. I play in Nova form most of the time and have done since lvl6.
So for 8 lvls there's been no evolution of my character as nothing has changed (except the number of slots in her powers)
Of course, things are going on in the background, but the point remains and I'm already looking forward to getting dwarf form and try being tankesque as well as blasteresque.
(\_/)
(O.O) Bunny: Our time is coming
(> <)
Yeah, I understand that too. I find Nova very good at what it does (ie strong and consistent AoE damage) and I enjoy the incomparable 3d mobility, but it does have only 4 attacks. Then again, same could be said about a scrapper with a toggle based secondary. There isn't exactly a huge amount of variety of what to do there, either. And with Nova, at least I can get into the other forms if I like.
I think a lot of people who find peacebringers boring are thinking about the forms in isolation. Nova is like Blaster with 4 attacks, Dwarf is weak Tanker, etc. If there's a blaster- or tanker-shaped hole in the team, you certainly can fill it adequately, but an already reasonably balanced team is where it gets interesting, as you can keep changing your approach on a per situation basis. At least that's how I would ideally like to play my PB. I'm not quite there yet, to be honest I already have so many tricks up my sleeve that I forget to use half of them. It certainly makes an interesting change to look at pool powers and think 'meh'
The only pool powers I have (at lvl 14) are from the fitness set - which are aimed at prepping the human form for when it becomes the dominant form in later lvls.
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(O.O) Bunny: Our time is coming
(> <)
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I'm not quite there yet, to be honest I already have so many tricks up my sleeve that I forget to use half of them.
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So true with my warshade, even after reading, reading and reading again Double's guide ! For example, I always forget to use my "semi"-nova which needs a dead enemy. But when I do it, Whoah
when i play any of the 2 epic ATs i just dont find them THAT much fun, am i doing something wrong playing it like i would any other AT, or is there a specail way your ment to play one to make them fun?