MomentIM is a very full-featured Jabber client. If you like that sort of thing.
In what I would consider to be staggeringly bad interface design, you cannot actually download MomentIM from the informational page I linked to above; that would be here, if you are so inclined.
Any reason to switch from Pidgin?
Not unless you need to send broadcast messages, i.e., send a message to multiple contacts. I believe there is a pidgin plugin for that, but it requires an explicit recompile allowing it, as it’s viewed as potentially abusive (spamIM!). MomentIM has this built in (as do some other Jabber clients, IIRC).
If you use the client at work, where you may want to tell all your co-workers “I’m going to lunch/upstairs/fix a PC”, it can be a very handy feature… otherwise, I’m using Pidgin myself. Adium on the Mac.