I am currently using a Samsung Vibrant.
Each OS has its pros and cons. Most businesses use blackberry because of it's email capabilities but you can always buy an android phone with a slider. (Samsung Epic for Sprint comes to mind)
To me the blackberry OS is just outdated: Web browsing is a complete joke and the apps are very limited.
I liked the iPhone 3GS when I tried my friend's iPhone. However, I wasn't wowed with the new iPhone 4. Has anyone held the device? Maybe it's because my phone is lighter than most phones but the iPhone 4 feels like a brick. Apple also decided to cut the screen size when they should have made it bigger. It was very responsive though. (although, my friend didn't have too many apps clogging the phone anyway)
It was hard adjusting to a touch screen for text and email. But, my phone came with Swype so eventually it was even easier and faster for me to text than it was with my old slider dumbphone. Everyone with a touch screen smartphone without a slider keyboard needs swype. Like me, you will complain a lot less after you learn to use it.