I use my netbook as my primary machine ALL THE TIME. I believe you can never have too much screen real estate but it is a tradeoff on so many things.
When I am running around I find the 10″ screen absolutely great, particularly as anything bigger would be a storage problem in my case/bag and also would not fit on the little tray table on the train/plane. The only time limit I have is the battery.
In terms of the home base however, it would be great if I had as much screen real estate as possible and I get this by plugging in and extending my desktop to a 19″ LCD. This is purely a comfort thing and would happen whether I had a 10″ netbook or a 15″ laptop – you can never have too much screen real estate. In fact, I would love to get a 30″ screen and connect the netbook to it – it can do that fine and look forward to when I do have one.
External keyboards and mice are the same on a netbook as for a laptop.
The netbook is just a multi-purpose processing device that scales to what you need it to do for standard information processing requirements.