I personally haven't due to cost. But two things to keep in mind if using a VelociRaptor:
VelociRaptors ship on 2.5" to 3.5" adapters that are used to keep the drive cool since they get very hot compared to 5400rpm and 7200rpm drives. So an active (fan) cooling solution should be used when using a VR drive. Mounting it inside the 360 HDD case would result in serious overheating.
VelociRaptors use the 12v power line, where most other 5400rpm and 7200rpm drives just draw off the 5v line so the 360 HDD adapter only feeds the drive the 5v power line. So you would have to wire up the 12v power source to the drive caddy.
Let us know if you ever do get a velociraptor working in your 360, I would love to hear about the load times and speed increases.