I have talked with Dilip so many times about this work and have used pLayer code myself - very nice!
Middleboxes are here to stay (whether purists like them or not) and datacenters typically have load balancers, firewalls, SSH offload boxes etc. in them. PLayer is a proposal for middlebox deployments and the proposal at heart, is quite simple. Configure policies, the set and sequence of middleboxes to traverse, at a centralized controller that then distributes it to the PSwitches.
I loved the way the paper goes about describing and motivating the problems - section 2.2 was very helpful in getting the magnitude of the problem and why existing solutions are totally ad hoc and ugly.
The solution is to specify policies that indicate the sequence of middleboxes to traverse for each traffic type. They talk only about middlebox types and not instances enabling fancy load balancing and resilience techniques. The details are well explained - seamless switchover on policy changes probably has taken up a little more space than warranted but no questions about its utility.
I had a question about the evaluation - it seemed to evaluate something that PLayer wasn't solving! The problem why PLayer came into existence was that middlebox configuration was a hard problem... and the ideal evaluation in such papers have to come from some sort of a testing on actual network operators. Evaluating latency, throughput etc. on a software router really doesn't indicate much.
Overall, a very nice paper...