I used to work in tech support in an IM company, and IM (unsurprisingly) was huge there. Each of the departments had a chat room, largely used for in-team communicaitons. The developers also had rooms per product, so there was a server room and a client room.

The chat rooms were used to throw in questions, but there wasn't an expectation of an immediate answer. The idea is the person who could best answer it would, when they had time. Unlike a forum thread, though, you didn't have to keep hitting refresh to see if there was more in there.

There were two things that really helped in that environment. 1) Everyone used full sentences, capitalization, and punctuation. No l33t speak or txt garbage. 2) People were vicious about setting their presence (away, do not disturb, etc). It was understood that your emergency was not their emergency.

Since support is interrupt driven by its nature, I didn't find it intrusive. It was handy to be able to ask people things while you were on the phone with a customer, too.

Something I rarely used it for was supporting directly with the customer. We only had two support engineers to support 80+ customers, and with the volume of even email traffic we had, it would have been too easy to swamp the tech support engineers into one-to-one IM conversations with customers that wouldn't read the manual.

I did use temporary chat rooms for big customer emergencies. Jabber has federation built in, so I'd set up a temporary chat room on our server, and invite in the customer, developers, maybe a sales guy or QA. We could then all discuss the issues, paste in logs, etc. all together. I'd normally have an internal chat room then, too, so we could discuss problems away from the customer.

A side benefit of jabber is that there are a lot of clients out there for it, and everyone was allowed to use the one they liked best. Or, heck, write their own client (which they did).