I've had this lens for a few weeks, obviously some distortion at edges (less with a cropped view) but much of it is as sharp as you can get. Bit of light fall-off at corners but nothing that photoshop vignetting section can't easily cope with... and I am using full frame, so there shouldn't be any dark corners on a crop sensor.
No real CA (colour fringing) to speak off, and that's going some on full frame.
You can't officially use filters on the front... although there is a thread on the odd metal cover/adapter that comes with the camera, so that could be worth pursuing if needed. Couldn't really go down this route myself as using this cover on F/F creates massive vignetting.
Focus sounds like a bee trapped in the lens, but it fine.
I expect there are better wide zooms out there, but at under £260 from W/EX it's a bargain (far cheaper than a grey import from HK costs!!)
cheers,
Andy