Nothing beats Yahoo messenger with push. Nothing
I must admit - been using iPush*t for a day or 2 now - not bad, needs a few enhancements though. Had a look at tetfree - not bad but seems bit "slow" in loading and displaying speech bubbles (just doesn't feel right).
Yahoo seems to be the best option still. Come to think of it, I can chat with friends on BBs too that way - so it's free messaging between iPhones and BBs
@Behshad, have you had any random disconnects from Yahoo or does it seem stable?