Dan Moren, wrapping up his three day test of the iPad as his only machine for work:

That’s perhaps the biggest change in switching from a Mac to the iPad: The technology becomes almost transparent. You are simply writing, or reading, or browsing. It’s all about the task itself, while the technology you’re using fades into the background.

That’s largely been my experience too, so it comes down to whether you like that or not — because it can be quite confusing. Moren wrote a three part series and you should definitely read up on them.