Another alternative is the current generation of MultiWii Mega boards with the 2.1 firmware release. A sensor upgrade along with the newest firmware has tightened up the performance of these boards and the ATTI, Autolevel, and GPS position hold all work. No they aren't as locked in as a Naza but properly setup they do work as well as an MK and for a lot less $$$. These boards will also run the MegaPirate firmware which allows use of the Ardu ground station with all of its functionality, or you can have the basics with the MultiWii firmware.
I started with the MultiWii over a year ago on a Quadrino board and firmware 1.7 which worked fairly well but clearly the advanced functions needed a lot of work. The board flew fantastic in manual mode and OK in autolevel, altitude hold was still a long way from perfection. Fast forward a year and I just recommissioned the quadrino with 2.1 firmware and now it all works. The icing on the cake is it flys quite well on a DJI F450 frame using the default settings. I could tweak a bunch of things to try and make it better but that's optional at this point from what I'm seeing in the intial test flights.
I recently finished a build using one of new generation Mega boards with GPS, same deal, it flys quite well on the default settings, just needing afew tweaks on the GPS to lock it down a bit better. Last weekend I put a cheap camera on it and did three FPV flights on the quad with MultiWii Mega, wasn't quite as solid as my Naza FPV quads but one thing it did much better at was FPV in manual mode. With a Naza you can set it up to fly well in manual in which case it usually has completely wrong settings for ATTI and GPS modes, it will oscillate and wobble all over. Conversely if you set it up to be nice and smooth in ATTI and GPS it feels like it's barely controllable in manual which makes it an either or proposition. Not so with tthe MultiWii, it flys smooth and solid in manual mode and then you can turn on autolevel and various other functions using TX switches that you define in the MultiWii GUI. End result is it appears to be a good and inexpensive alternative to the Naza for FPV, even having RTH and failsafe modes that actually work. I've tested the RTH and found it to be almost as accurate as the Naza, have yet to try the failsafe.
The best part is the boards with GPS can be bought for less than the cost of just a Naza GPS unit, I paid around $140 for the MultiWii Mega with GPS, there are even less expensive units available from HK, Goodluckbuy, and even a couple US based vendors. Software is opensource and well established, though don't expect it to be as plug and play as DJI but with the GUI andthe current firmware it's not the tuning nightmare that MultiWii used to be. I think it works well enough that I may use this for all future builds for FPV and sport flying and I'm considering trying it on an APV hex, I think it can do the job.
The future of multirotor flight controllers I believe is with the open source systems, eventually the capabilities will match what the big boys are doing now at a fraction of the cost in hardware.
Ken