I've got an 8 gallon Megapot that I normally do 3 gallon batches in but have done a couple of 5 gallon batches with no problem. I was lucky enough to get mine off Craigslist for $50 and it came with a thermometor and ball valve. I've very happy with the Megapot, however, those Blichmann kettles sure are nice.
Edit: Although I didn't have any issues with the few 5 gallon batches I've done, when I start to move to brewing mainly 5 gal batches, I will be getting a 10 gallon kettle.