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It works over cell towers. (I don't really feel like going into the technical details, and I hope that not what your looking for) But basically, you get your "last mile" connection over cell signals. From the cell tower on however, it is a normal digital line. Think of it as a cable modem. The only difference is your "cable" is a cell signal.
As far as reliability, it is as reliable as cell service. I know that sounds kind of like a dodge, but cell service in different areas can be a crap shoot. If the places you normally visit give you good cell service, then you should have no issues. Keep in mind that good reception with AT+T does not guarentee good service with Qwest or Verizon etc. As far as logging in anywhere. No not really. It all depends on peering contracts and providers and such. If you get the right provider for the areas you frequent, you will be happy. However, research your providers and where you are most likely to use the service and make sure thier coverage maps line up with your usage.
As far as speed. If your only web surfing, emailing, and such, you have all the speed you need. Downloading huge files, gaming and file sharing however, you will be less than impressed in your speeds. I haven't looked into it at all recently, but i believe it is at best a 1 meg down pipe, with a much lower upstream speed (128k?). I could be wrong on this however, so do not take my word on the exact speeds. Just keep in mind the speed they quote you is best possible, and your real world speed WILL be lower (on average).
Pricing however, I can not speak about. I never looked into it, and I do not use those services. However in my job (network admin) I deal with a lot of customers who do use the mobile WLAN connections, and hear their complaints incessantly
<edit> Also there are some new products and services coming out (hopefully) this year that will increase speed, reliability, and coverage.
Wiimote's right on this one; the technology hasn't quite caught up yet. There's satellite based internet that will pretty much work anywhere, but it's still subject to bad weather and it's also very expensive - like a couple hundred a month - which pretty much makes it prohibitive.