The addon flat lens corrects the blur without having to actually replace the curved lens? (like I've seen some other options do)
The Big advantage of this design is that it keeps the original Gopro housing seal independent of the Blurfix adapter which makes its own separate air chamber to fix the Blur issue, if for any reason you had a filter failure or you dont quite set the filter properly then flooding the blurfix wont effect your camer at all.
How is the blurfix attached? Permanent once attached but per the website you can remove the outer lens/oring and basically use the stock case curved lens for dry picture taking?
It attached by replacing the original black ring on the gopro with the Blurfix adapter, you still use the original oring which seals the main housing. Some silicon sealant is used to keep the Blurfix air chamber water tight but this can all be removed if you need to so its not permanent but you wouldn't bother taking it off unless you really had to for some reason.
To attach a different filter (ie UV) for picture/video taking above water, do you leave the blurfix lens on and just put the filter overtop?
The Blurfix lens itself is the filter, so the stock lens is a zeikos clear UV 55mm filter ground down a little by SRP to be very low profile so it avoids vignetting in the wide modes of the gopro. Stacking filters would cause vignetting as the Gopro does have a 170 degree FOV which means you dont have much room to work with to avoid vignetting. Here is a pic I made to explain basicly how this works and why filters cant be too deep when using a gopro.
This is a blurfix with a Filter provided by SRP if you imagine a line going from the gopro lens extending through to the filters inside edge, My line is just an example to demonstrate this but in theory the angle where these 2 lines would meet should equal the gopros FOV angle of 170 degrees. These SRP filters are vignette free in all gopro wide video modes with only a slight vignette in 960p and wide photo but this is only a few pixels in the corner. Any filter that would extend beyond this red line would cause some vignetting in the wide gopro modes.
Here is an example of vignetting in the 960P video mode with URpro swcy filter
Here is the wide photo mode in 11mpx with URpro swcy filter
All other modes are vignette free with this filter
Changing filters is simply done by unscrewing one and screwing in another, the blurfix takes any 55mm filter but to avoid vignetting you need one that's very low profile which is why SRP actually grind down all the filters they sell a little bit for best performance. For above water use you can just change the filter or if you dont want to use a filter you can just use it with no filter attached and it will work the same as a stock gopro housing.
You may be able to find other 55mm filters that are low profile enough but most likely they may all need to be ground down a little but I only use SRP provided filters having the clear UV, Urpro underwater filters and I have some marumi CP filters for above water use.
Most standard low profile 55mm filters should be vignette free in 127 fov modes but will most likely vignette in the 170 fov modes.
The Gopro OEM fix uses a very similar concept to the Blurfix and is the reason why its so big, the square design is so that it gets vignette free in all modes. So this fix will not have the vignetting in 960p and wide photo mode but if you see my example pics the Blurfix vignetting is so sligh in these modes I dont see it being that big an issue.
Depth rating of the Gopro version should be 60m which is a deeper then the Blurfix which most filters are rated around the 40m mark but this list is available on the SRP site of what they have tested each filter to.
Main disadvantage I see is the square design which will make it a bit trickier to use filters as standard round filters wont work too well if at all.