I've never owned one but have a friend who uses one (although I don't recall his model). He observed that it works reasonably well for stuff that's primarily going to happen server-side (shocking, I know) but that he prefers a conventional OS X / Linux / Windows system because he often finds himself wanting to do things locally that should be easy (i.e. scripting, little GUIs) but actually turn out to be hard. It's 90% there but that last 10% makes it not really worth it for him.

He also observed that modern MacBooks and good Linux machines, like that 13" Dell, can be bought for $1000 – $1500 and last for years. For those prices he'd rather buy, either new or refurbished, and have less hassle.