Of course, VMware could have overpaid -- it wouldn't be the first time a major vendor has gambled on an emerging technology and lost, and it's betting on a company that is five years old and just came out of stealth mode.
appears to have legs. After all, one advantage is that it commoditizes networking by having virtual switches run on standard servers. As this article points out, it essentially gives IT managers a view into the network just as VMware gives a view of compute resources.
If you haven't thought about SDN yet, read this analysis of VMware's strategy and look into it.