I use verichat. Use it everyday all day with no problem. Its a little slow at showing away messages but I like it. Also tried yak on. Liked it but it does not show away messages.
I've purchased Yak-on and it's great.. no problem with this app yet. I'm currently trying out IM+ the new version.. it's preety sweet i love it, even tho i get disconnected from time to time it is still a good IM app. I hated Ramble because I always get disconnected.. but hey it's free.
I use GoogleTalk, because it does not use SMS as the transport. So, there's no additional charge (I use T-Mobile's unlimited internet package on the 7105t).
Even though it is not on your poll, I jumped from Verichat (subscription expired) to WebMessenger. Like it a lot- low batt use, good connection and features. About as good as Verichat was form me.