Well, I have a Macbook alum. and I use my 3GS mainly as an iPod and for frequent web-browsing, emailing and that kind of thing.
When I don't have my laptop with me, I find it very uncomfortable to type for too long on the iPhone as my thumbs start to ache after a while, and also while it is fine to browse the web, it's not ideal. An iPhone 4S wouldn't resolve this problem, I think.
Also, it would be cheaper for me to get an iPad2 than to change my handset as I'm not yet eligible to upgrade and I'd have to buy a carrier-free iPhone which is more expensive than the iPad2.