The first thing I'm going to say about this book is consider getting the Kindle edition. Not because its cheaper, but because like so much of Stephenson's recent work, this book is, well. Huge. It's over a thousand pages, making the hardcover not something you want to haul around unless you're a two hundred pound weight lifter.
Also, Reamde is not science fiction. Let's get that out of the way first. You'll find it on the science fiction shelves solely because it's by Neal Stephenson. Possibly next to Cobweb, which is also not science fiction. If a mainstream thriller writer had written this book, it would be on the thriller shelves. It's a techno-thriller, plain and simple, and is much more about people chasing each other with guns than new technology.
Is there technology? Yes. There's a computer virus and some of the action takes place in a sophisticated MMORPG named T'rain. As far as I know nothing quite like it exists, but I saw some of the ideas in MUDs twenty years ago.
Reamde is not science fiction because there is no new technology. Everything in this book either exists or could be put together without any scientific innovation at all. It's not cyberpunk either. It's a long, complicated thriller with a twisting plot and some interesting characters.
If you accept it at that premise, it's good. Not great, but good. It's not Snow Crash, but it definitely has its moments and I personally preferred it to the conlang intense Anathem. As thrillers go, it is a little slow and I'm not sure it needs to be 1000 pages, but it is a lot of fun. Just don't take the hardcover version on vacation. You might end up paying excess baggage fees.
I'm giving this one three and a half stars.