Blue Crayfish
I have a blue crayfish (indeed not a lobster, blue lobsters are marine animals and extremely rare- aquarium stores often incorrectly market them as lobsters because it sounds more exotic than just a common creek dweller, mine was a "lobster" when I bought it) in my 55 gallon tank. When I bought him (in March '09) he was about 2 inches long and has molted 5 times and grown to about 5 inches and a brilliant blue, however he was never even close to the color of Bluestman's crayfish at the beginning of this thread.
Several people have posted comments about how crayfish will kill and eat all of your fish. I would say this is an exaggeration, but there is some truth behind it. When I first introduced my crayfish literally within minutes it caught and killed my senegal bichir (an eel). The senegal was 3 inches long and the crayfish managed to eat the entire thing-earning him the name Pulpot after a tyrannical dictator. I think, though, that the only reason the eel was killed is because of its poor swimming ability and it was sort of a bully and always harassed the other fish; I think Pulpot decided he was not going to be harassed. Over the next couple of months it ate a few other fish (a couple of tetras and a guppy maybe something else too, can't remember) but I didn't see these kills and I believe they were mostly already dead before he ate them. Over the past 6 months or so no fish have gone missing and the crayfish has become very tolerant of the fish. My rainbow shark often shares Pulpot's cave and even rests on top of him. In the picture you can see my pink kisser sharing a meal of pellets with Pulpot and it survived just fine.
Overall Pulpot is probably my favorite tank inhabitant and well worth the loss of a couple of fish. Any time I have guests over they are fascinated by his bright color and love to watch his many appendages search for food and he's always a good conversation topic. People will have to choose for themselves whether or not to add a crayfish to a community tank, but for me it has worked just fine.
A bit more about Pulpot. He likes to dig so I recommend fine sediment and hardy plants as he often uproots plants with his big clumsy pincers. Also, as already stated in this thread, crayfish are escape artists and will try to climb anything in an attempt to escape so keep your tank sealed up tight. Another good characteristic is they are easy to keep. Their exoskeleton makes them nearly invulnerable to external parasites and less sensitive to changes in water quality.
Anyway you have a very cool crayfish, Bluestman, I hope mine gets that large.