I recently purchased the FlashChat script because it looks so nice and is so affordable. They don't have an integration module for any ubb products yet, so I stuck some authentication code in the chat page to grab the username and moderator status.
It operates flawlessly with 3 or 4 people in it, but when I stress-tested it with my staff, getting about 20 people in a room chatting back and forth vigorously, the lag went up to 15 or 20 seconds inbetween lines for everyone in the room. In addition, graemlin/smilie codes are interpreted on the fly, and a whole line of "

" (x 30) will freeze everyone's chat window and cause them to time out and have to wait for the default time-out period to log back in.
I was really sad that it failed under moderately busy chat conditions, since I reeeeeally liked the look of it.
I currently run the commercial version of GlobalChat (formerly iChat) and it works well, but the main client is ActiveX and newbies are all frightened off by dire warnings about "If you allow this ActiveX control to install you will become sterile. Continue?"
I'm not especially fond of the Raidersoft chat that has been so successfully integrated here... does anyone know of any other robust chat servers that will merge with ubb AND handle crowds of 100+ users without choking?