I don't think I wanted this I was was just looking 8.1 GDR2.
so I installed insider and got windows 10 10.0.10166.0 is this stable if not can I revert back to 8.1 and just 8.1 GDR2
it depends on how you use it. It was OK for me for a week and then the bugs kept bugging me too much, specifically with missing notifications with my MS Band. Otherwise I think it is pretty stable build.
You can revert back to 8.1 by using the WP Recovery tool (
link) and flash it back to original rom.
ps: many were able to update to 8.1 update 2 first with insider program, but for some it also went straight to W10 mobile (e.g. for me there was never an option to go to GDR2 via insider program).