Without being able to replace the messaging app on an iPhone, I don't quite see the point. But I also don't see the point of things like WhatsApp so that shows how informed I am. I'm also not one of those people trying to duck an unlimited texting plan.
I might install it and kind of use it as a Google Talk app, but unless it starts accepting messages via my phone number (not my Google Voice number either, though I understand that, in the end, that's Google's master plan anyways) or there's just a dramatic shift in usage that I can't foresee, it's going to be just another messaging app.
pm, the messaging apps I use all use push notifications, so even if the app dies, you still get notifications of new messages. Haven't had a chance to use the lync app yet, as we're still on ocs2007 without external access, but if it didn't do that it would seem like a huge oversight on Microsoft's part.