I wouldn't buy into this IPO. The problem is rackspace doesn't really have much of a moat and the space is highly competitive.
The space has a moat though - it takes a huge investment and long time to become competitive in this low margin space. Rackspace has shown themselves to be a market leader within this moat.
Their capex looks to be ~$2-300MM. That's approximately 1% of what AWS spends.