I've been using the Acer Aspire One with built-in 3G connectivity for a few weeks now. With that experience, I can tell a few stories about the Aspire One.
The first story is that this is a perfectly useful very small (roughly 10" x 7" x 1") notebook computer. The keyboard is just big enough for touch typing, and the screen is big enough (8.9") with just enough resoluton (1024 x 600 -- though 1024 x 768 would be better) that web sites are usable. The weight is low enough (2.8 lb.) that you aren't burdened by carrying the Aspire One. The battery life is pretty short (in heavy use, about an hour), but not ridiculous.
So, if having a very small notebook is important to you, the Aspire One will be something of a breakthough -- not for functionality but for price. That's because small notebooks have typically been pretty pricey. Take Sony's TT series: with an 11" screen and weighing 2.9 lb., it is similar in bulk to the Aspire One. But the Sony costs $2100, compared to $349 for the Acer (without the $60 per month 3G bundle that discounts the computer to $99). Sure, the Sony has many upscale features but this illustrates the price gap between fully featured small notebooks and de-featured but still usable netbooks.
The second story you can tell about the Asprie One (and probably many other netbooks), is that this generation of product doesn't quite have the magical mix of features to make it a new kind of device. A new kind of device, for example, would be small enough to always have with you, fast enough to quickly access small bits of information, and connected enough to work almost anywhere. Sort of like an iPhone with a bigger screen and a more usable keyboard. The Aspire One with 3G is close on all of this, but not close enough. Mostly, it is too slow. Not at processing, but at booting up. A new kind of device needs to be always on (sleep mode) and then come alive in a few seconds. The Aspire One doesn't have that kind of battery life, so you have to turn it off. Boot time is then, well, Microsoft XP long -- which is to say very long.
The Aspire One also isn't quite small enough to have with you all the time (or at least I didn't form the habit). Part is this is size, but just as important is form factor. Somehow having to open up the clamshell screen is a deterrent to use (the machine seems imbalanced, like all notebooks, when not on a table).
The 3G connection (plus WiFi) was very handy, especially when in true 3G mode. 3G isn't available everywhere, but some cell connection is usually available. If you are sending data or working with normal websites, this is perfectly usable. If you want to stream video or audio, I'd expect to be disappointed (not Acer's fault, that's where the technology is today).
All-in-all, I like the Aspire One. A more radical approach would be even better, but that will probably require Apple to step in. Why we have to wait for Apple to pioneer new categories is a question for another day.
In case you missed Tom's unboxing photos, they are here: