Pretty much everyone in the open-source arena (Ardupilot, OpenPilot, Paparazzi, PARIS/WiiCopter) experiments with various filtering approaches and sensor fusion strategies.
One interesting challenge is that more sensors increases cost and hobbyists are often cash-strapped (especially after crashing a lot!), so sensor fusion isn't as heavily pursued as adaptive control and basic filtering strategies for 6-axis input.
I had a lot to do with the question of "where the hell are we & how the hell do we get to where we should be" in real time and to fairly tight tolerances (at scale) - sadly/luckily at the time it was left to the pilot to follow the blinking light rather than deal with the mechanics of moving control surfaces directly.
The main thrust of surveys like [1] though was enhanced final product accuracy through sensor fusion strategies, using calibration flights to tune filters to remove induced magnetic field effects from aircraft headings, using lidar, microwave, accelerometers, differential ground GPS, temperature, pressure, and humidity to get good estimates for actual height clearances, estimates for the mass of air column, etc, etc, blah, blah, old guy stuff.
That's a single survey of many, performed over decades, and it was interesting in the evolution of processing to watch it move from post processing to in line processing. Which of course prompted the question.
The paparazzi (seriously france? this is for drone RC craft to snap pics of celebrity boobs?) project pages look interesting, so I'll dive into those.
[1] Fiji, 1997 "A combination of helicopter and fixed wing aircraft was used to fly the survey with a total of 160, 000 line kilometers of data acquistion (~80,000km per aircraft type)"
http://webcache.googleusercontent.com/search?q=cache:http://...