I've done pretty much everything on my N900 but will admit its not a full blown replacement.
I guess its difficult because I don't own a netbook (a 17" Laptop instead) but I find the N900 screen a little too small to ever consider it a replacement. But Ill also say I've never WANTED to browse the internet on a phone before and I do quite often with my N900. I'm very rarely a mobile gamer but I've found myself playing Angry birds whenever I have an idol moment.

And the thing thats most important to me. With phones in the past they are just phones with a little extra, But with the N900 I feel as though I'm using a small computer.