There've been a couple of threads about them here... dig them up.
Consensus seems to be that they're good but not nearly worth the price.

I'm a fan of bolting a drilled cog to a disk hub myself (there's also a thread or 2 about that).

Level promised to make a freewheel for the hub which wd have made the system more versatile than a disk bolt-on solution but they never managed to make one and it looks like they never will... and it wd have been sketchy anyway... proprietary=bad