Normally I avoid CTPs like the plague, but I'm interested enough in this one that I installed the first CTP of the Velocity caching engine over the weekend.
Haven't tried coding against it yet but I had a bit of a play with the command-line admin console and read some of the help.
The most obvious missing element right now is a CacheDependency object (+ subclasses viz. SqlCacheDependency) - I've been trying to think of a way to chain this and the ASP.NET Cache together to retain the ability to have dependencies butI haven't come up with one yet :-(
I'm intrigued by the idea that you can lock and unlock cached objects, though since in the finished version it should run across multiple servers I guess this is a way to avoid race conditions.
Edit: Something else missing I just discovered is that the service out of the box doesn't survive a restart i.e. you have to go into the Admin console and issue a new 'START CLUSTER' command. I've now marked the Windows service as 'Automatic', not 'Manual'.