It's obvious isn't it? GMail's very nice compression of threads makes email as efficient and compact as IM.
It's secure. File sharing comes naturally right inside GMail. Images are inlined in GMail messages. 2.6 GB free file storage space. Permanent archive with efficient and simple search. I frequently conduct 30-50 email conversations in groups of 3-5 people in GMail. Feels very much like IM.
That, being said I think Campfire looks nice and fits into the "simple language, all-in-browser" category replacers 37Signals seem to like doing. If I didn't already have a superior free version I might just buy it.