Ok..It defiantly isnt a replacement to your everyday laptop or desktop..
A large iPod Touch basically..They have some pretty damn good apps to offer, nd depending on if you get the wifi or 3G version (or both) you will be limited to what you can nd cant do around the place..Yeh like Rach said, it sucks that they cant be used as an external harddrive.. The processor in them are alrite nd they are pretty quick..But Apple have labeled them as a "laptop" but i dont reckon that they are anything like a laptop..Good for reading books too i guess if thats what you like..
I dunno, but personally i think i would rather save a bit of $$ and go for a notebook laptop..(as they are around the same size) and there pretty handy and do everything a normal computer does..You can pick them up for around $600 nd they are resonable too..
So yeh, its up to you, but i think of them as a large iPod Touch..
Best of luck but mate!