I have the F4Rs (2012 tubular version) with the FFWD front hub and Powertap G3 rear.
Weight with stock hubs was 1250g IIRC and now the weight is around 1325g. Not too bad for a 38mm rim with a powermeter.
Got them in an insanely good deal along with some Ultremo tubs. Total cost, including P'tap, rear build, and tubs was I think c £1350.
Anyway, I'm impressed. I also have Boras and 202s (tubs) and they compare very well against those. Not better, but certainly not inferior when you consider them all in the round, if you'll pardon the pun. Not quite as agile feeling as the 202s but then they are 225g heavier.
Stiffness is good if not stellar - no brake rub to speak of, and pretty much comparable feel stiffness wise to Boras.
They have remained perfectly true after 3000km or so, including 1000km or so of commuting across London.
FFWD hubs seem fine to me - spin very well in the stand (obviously there is absolutely no correlation with drag under load
The braking with Reynolds Blue pads is very good indeed. Better actually then any other combination with carbon rims I've come across. Lots of bike.
Only thing I don't like is 20 spokes front. Seems at bit OTT and IMO detracts a bit vs. a 16 or 18 spoked front.
But all told these are very impressive and hold their own against more expensive options IMO.