My experiences with gmail are very positive. I have about about 10GB of mail across 4 gmail/apps accounts with about 100k messages, and the performance is almost always instant (search, send, read, file). Furthermore if I travel to the other side of the world (which I do 3-4 times a year) then it continues to offer a quality level of service.
I've never seen any in-house system get close to what I get from gmail.