Here is a more recent post with benchmarks of different GUID algorithms in SQL Server, the author has a different outcome.
In his benchmarks about index fragmentation, COMBGuid is not really much better than normal GUIDs.
http://microsoftprogrammers.jebarson.com/benchmarking-index-...
It does look like the sequencial ids (not the combs) does about 50% better overall, and the comb from nhibernate does about 10% better than just random uuids .. this is in terms of paging and indexes.. total time to run the test isn't bad, but that's just the insert time. If you're generating from your application layer and storing in SQL Server, looks like the best option is UuidCreateSequential.
