My experience is that people don't want to wait around on IM to get their question answered-- they prefer to drop an email and get a quick-turn-around-time email response.

I've done both chat and email reference, though not IM-- but answering a question via IM or any other live chat takes longer than most IM users really want to wait while online. If you are used to working with 20 AIM windows on your desktop I guess it might work, though.