Reading through this thread, I currently have a 9 gallon aluminum pot and no off-flavors from using aluminum. I am currently in the market for a larger pot to increase my boil volume and avoid boil overs for a 5 gallon batch. I would also like a ball valve for easier transfer to the fermentor, and volume markings would be helpful. I happened to stumble upon this:
http://www.txbrewing.com/15-gal-stainless-kettle-assy.html
It comes with welded ports, ball valve, thermometer and gallon markings for $180. I was previously keeping an eye on a Bayou Classic that had gallon markings and only a weldless ball valve for $120-140 on Amazon with free Prime shipping, but I think I want to pull the trigger on this kettle. Seems to be a good value unless there is something I should be concerned about.