i have a tippmann
Performance - an out of the box TM will be better, unfortunately the tippmann suffers with a crap hop design. that said it can be made good, i currently have some 3d printed nubs from shapeways. also out of the box you will have to set its ROF, some peoples gun really struggle with this, but if all else fails clean it! then it usually sets ok. Don't expect massive ROF, you are going to get 10 - 18 max, most people get 12bb's/sec, which isn't a game breaker in my view. once you get The hop sorted and your FPS dialed in, range should be comparable to most guns out there.
Air cylinder - I suffer with back pain at times, i find its a nice counter balance, and to be honest there not that big the 48ci ones (0.8L) that said the tank can soak up a bb hit meaning you may take a hit but wont know about it etc, that said a plate carrier can do the same.
Parts and service - Atm the spares kit you can buy is the easiest to get hold off, that said the tippmann dose come with some spares. though atm long term test are looking very favorable, with some sites swapping there rental guns for tippmanns. as for service, they are really easy to work on loads of video's on youtube etc.
Down sides vrs AEG's - it can and it will shake any accessory to bits they do have a looooovly kick the only other one i can think off is having to have you air tank refilled
How long dose the tank last - thats a player/site specific thing, do you spray and pray, or semi all the way??? According to tippmann around the 3000-4000 bb's per 48ci 3000psi tank. i tend to get mine refilled after every game (usually around £3 from dive shops)
Do skirmish sites have any issues with running tippmanns? - No but you will need to get hold of a "tournament locking pin", it replaced the rear locking pin and allows a tire-wrap to be put through the hole to show that you gun hasn't been apart etc, there usually around the £12 mark.
Another excellent source of info is the "tippmann airsoft owners group" on facebook.
I hope this answers your questions .