This program, QuickIM, works just like the desktop version of MSN Messenger. Of course each program has it's flaws. I think it would be great to have a pop-up window from your Home screen but hey, its a cellphone not a computer.
It was easy to setup and easy for me. I installed it on my Blackberry 8300 and I was good to go. Probably the best advanatage that I have found is simply being able to have QuickIM run in the background while I am doing other things, but also being able to use emoticons. Just like home. I recommened it! Its the first program on my list.
Check it out for yourself, at least download it and give it a try quickim.com
Ramble has been my program for AIM - I've used it on every blackberry I've had and it works great on each - and it's free... no bells and whistles, but it works for what you need...