I've had a 3g mobile phone for a few month and find the near broadband speeds of upto 384kbps to be the biggest benefit.
It works great with my pda and my works laptop. I have however found a downside. As you pay for what you download, and not for how long you've been online, it's easy to get carried away and before you know it you've blown your monthly bandwidth allowance.
This was made painfully clear when I got my Orange mobile phone bill today. I used my 3g connection quite a lot when I was on my training course the other week. One evening I'd been online for about 10 minutes checking emails and updating my blog when I noticed I'd apparently downloaded over 20Mb of data already. This was thanks to windows update using my very expensive bandwidth to download a shedload of updates in the background.
As I'd gone over my monthly quota already that download cost me over £20 at £1 per Mb!!