Mini review - I've only tried the main unit, I have not connected any of the accesories as yet:
Took it out in some pretty awful weather yesterday for about 4 hours and it was fine, so I'm hoping it's reasonably weatherproof. USB port is well sealed, buttons are rubberised and sealed too.
Screen is easy to read & very customisable. I've got it displaying 6 fields on my "home" screen and each one is just about readable for me, but I'll probably knock it back to 4 in the long run though.
GPS fix is quick & accuracy seems good.
There is a lot of data available if you want it, this thing is jammed with sensors it seems!
Mount is simple and seems robust - no movement whilst I was out on the bike yesterday.
It seems reasonably straightforward to set up & you can configure the unit through the software easily enough (one good bit about the bundled software, see below for the bad).
Not so good bits (not bad, just not so good!):
Manual is rubbish - but the Timex version is much better (same unit)
Bundled software is a bit naff, but it works. It's a bit clunky, but once you've used it a couple of times it only takes a couple of minutes to connect, download and save the data as a GPX (you can save as GPX, NMEA, CSV, KML and TXT). I think you can use other software, but I haven't investigated that yet. I tried the Timex software, but that didn't seem to work - maybe I need to try again.
I'm not convinced of the accuracy of the internal barometer (elevation data), will be having a play with that to see if I can improve it.
Buttons are a little difficult to press with gloves on, and do need quite a positive press.
Additional bike mounts might be difficult to come by, which is a shame. I feel a spot of bodging coming on!
So far so good, we'll see how it holds up after a few months of riding.