I just got a set from a friend of mine who was racing on them for the last few years at Team Sky, but who is no longer riding them this season.

The first impression is that they are very different from the 'Q'-Rings. They have a much more significant 'ovalization' factor than the 'Q'-Ring and are harder to set-up perfectly. They do get there, eventually. I found that they are very sensitive to the derailleur height setting and seem to benefit from a slight toe-in (on Di2, anyway). The other thing I noticed is that the Di2 derraileur really pushes the ring around.

I got them fairly late in the winter and will play just around with them during the season. I'll wait to give them a real try next winter.