Ventajas
Great culture, especially at the lower ranks. So many places espouse a great corporate culture and the rules they live by, but it's actually true at Rackspace. For a small company there is a big effort to plan well and use accepted models of getting things done, rather than trying to invent the wheel, which I really appreciated. People are the most valuable asset, and I miss working with more folks from there than any other job I've had.
Desventajas
Quite a lack of professionalism in the lower ranks, although that was changing in the time I was there. If you come from a big company, this will surprise you and take a while to get used to, but the postives on the cutlure side help alleviate that to an extent. Not a great level of technical understanding at senior mgmt levels, so they miss the point in that area sometimes. Although it is primarily a service company the underlying infrastructure has to be sound.