Re: how stable is the win10 preview?
is it stable enough to use on your primary device yet?
I use it as my primary OS on my Surface Pro 3 and I haven't had a single issue with it yet.
I also have my desktop dual-booted with 8.1 and the 10 Tech Preview, but I haven't needed to use the 8.1 OS on their either. I only had one Blue Screen on the desktop with 10 when it was first released (before any of the new builds have come out) and I was really TRYING to crash it.
I find it perfectly stable, however I am also a developer and IT 'pro' so YMMV. If you are comfortable with troubleshooting drivers, know your way around the Device Manager, and aren't afraid of the registry (not that I've HAD to go into any of these sections) then I say go for it. Otherwise, I'd say hold off until later this year (May-June timeframe) when things should be much more stable for the average consumer.
I will also be installing it on two of my phones (my daily driver - a One M8, as well as the 520) when it is available, and will only keep my 1020 on 8.1 as a backup in case something goes horribly wrong