i've tried both im+ lite and nimbuzz. Nimbuzz is great, the only downside is that you have to sign up for a nimbuzz acount (really simple though, and can be done rihgt inside the app)
im+ has simple text chatting, that's it, nimbuzz supports smileys, personal message, and also adiuo chat (if you have a microphone). Nimbuzz also supports skype, which most of these apps don't, and it has contact search, so if you have a lot of contacts, definately go for numbuzz
The best one that i have ever used is Fring it allows you to talk on many other different services. Although if you want something really simple that can go in landscape mode for a larger keyboard i would go with im+