Don't do it. I thought the same thing and got rid of my 3GS for a BlackBerry Tour. I regret it every day, and BBM is annoying and overrated. It's cool for like the first two days, and then it gets irritating.
I am anxiously awaiting the release of the next iPhone so I can finally drop this piece of s**t BlackBerry.
The only reason I'm glad I have a BB now is because I'm a lot more excited to get an iPhone back again, especially a new generation one, which should be awesome.
This.