It seems to be an issue that comes and goes. It might take off with a level horizon, spin it 90 degrees and it might tilt a bit. Fly around, and it might go away. It is annoying and DJI should be aware of it. It is almost certainly not a hardware fault (that will be corrected in the Phantom 3 anyway). If it can be fixed, it will likely and hopefully be fixed with firmware update.
Doing the full IMU calibration and also calibrating the gimbal does lessen the problem significantly. The Phantom 2 Vision+ would also sometimes do it. Except it was less visible because of the wonderful 720p video link to your iPad with the Lightbridge system and because the Phantom 3 uses a rectilinear lens (which is awesome) rather than the fisheye lens of the Phantom 2 Vision+ or GoPros.
It is an issue and a nuisance and hopefully DJI can improve upon it with a firmware update. It shouldn't hold you back though - as there likely will not be anything better in this generation of the quadcopters - and any possible fix will almost certainly be via Firmware-update.
I've done some correcting of a slightly tilted horizon in my photos and videos. It does not lose very much. Especially if you get the Phantom 3 Professional and do videos in 4K there is a lot of pixels to retain the quality of the image to a degree as I will say it is virtually not visually detectable that the video was rotated a 1 or 2 degrees.
If you decide to get one of these great machines (I have a Phantom 3 Pro (flown 38 flights so far - and hundreds of flights on my P2V+)) and you would like to buy from DJI directly (think they might have the quickest delivery time currently over Amazon etc.) I sure wouldn't mind if you'd use my DJI-publisher-link to buy it  That would give me an 8% credit to buy accessories etc. from DJI 
- Buy your DJI-gear, batteries, cases by using this link - it'll help me out  Phantom 3 from the DJI-store
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That would give me an 8% credit to buy accessories etc. from DJI 
