Two things I'm having some fun with tonight (but that I don't think will actually be using much) are the Google/Talk and Yahoo Messenger Blackberry clients - which are not commonly available in Europe, but which I was able to download today from the RIM site for some reason. Before today, I usually got a message stating that either was "unavailable for your region" (go figure).
Although both obviously share a lot of UI design with the built-in Blackberry Messenger (there is, after all, only a finite number of ways to draw buddy lists and the usual red-vs-blue scrolling chats on a Blackberry) both have subtly different ways to go about their business where it regards notifications (finally, a real use for the Yahoo "buzz" feature) and overall integration.
Also, there seems to be some trouble with the Google/Talk client - it doesn't seem to like complex passwords (probably an input masking bug in the password field), so I just went and created a new account to use it. As was to be expected, it can talk to other Jabber services just fine, so I will probably keep it around for a bit.
But neither strikes me as something that is fundamentally revolutionary on a mobile phone - maybe I'm just jaded from having tested too many clunky clients, but SMS still feels more useful, as well as more universally acceptable.
After all, everyone I might want to chat with (or that I feel that might need to reach me when I'm mobile) has had an SMS-enabled phone for years, and the Blackberry handles SMSes in a nice "conversation" view.
Sure, you have presence, and a roster, but the store-and-forward mechanisms of SMS enable deferred delivery, delivery reports, and, above all, they don't impose on you. No "typing" notifications or popups to build awareness that the other person is waiting for your reply - in fact, there's no real pressure to reply instantly (at least not culturally, and not in my age range here, although I understand that younger people delight in ping-ponging dozens of inane messages between each other).
As far as I'm concerned the less interruptions I have, the better, and mobile IM feels like a major disruption just waiting to sneak up on me.
Still, I might have a go at an MSN client like the ones I used on my Nokia/9300 - if only because I somehow associate it more with a "work" context, and it can be very useful to swap snippets of info when I'm away on meetings or stuck in a data center without the right info to set up a machine (i.e., when I need stuff that the other person can copy/paste to their chat window).
Some people have hinted before that IM is something that only took off because some countries took years to catch on to SMS and pre-paid phones, and despite my not subscribing to that narrow world-view, I still sense some truth in it - at least regarding the recent hype surrounding mobile IM in the US...