I thought I would post my experience here. Fair warning it is a little bit of a read covering two systems installations.
Firstly because I was wary I installed Win10 PRO 64bit on my laptop which is getting on in years - HP DV6. Installation went smoothly with no hiccups using a bootable USB and running "setup" from there. As a note, I decided to wipe everything and keep not settings or applications as I wasn't happy with the "bogged down" running that was occurring on my 8.1 install. All things completed and upgraded I gave it a whirl, and I hit my only snag - my display adaptor is an on board ATI chip and it had a long "initialising" time as @Wendell mentioned recently in another post it was conflicting with the other integrated Intel graphics. The solution was disabling the Intel graphics which allowed the ATI chip to run without a hitch.
After this was done everything else on my laptop just "worked" so this gave me confidence to try it on my main rig.
So firstly it was originally on 8.1 home 64bit, with a Intel 4790k, corsair 16gb ddr3, 512gb SSD, NVidia gtx980, corsair rm650. I did all my due backups and used game save manager then clean installed 8.1 Pro 64bit. No dramas. I then USB upgraded to Win10 Pro 64bit. No dramas, none, not at all.
Ill expand a bit on that though - I reinstalled every program I needed, restored game saves, updated all drivers (driver booster for the win) ran windows update (twice for good measure) and tried to install the graphics driver - which failed, I though this would then be a huge problem but nope - a quick restart and then it loaded up fine.
I went through every registry hack, gpedit, and setting(control panel) I could to remove or disable Windows made tracking obsession (Cortana you died at the end of Halo 4, you don't need to make me have those feels again) I ran 3Dmark as a tester and only noted a 2% performance increase which is negligible and could be put down to the clean install. Display fusion, Battlefield 4, ARK, ESO and Fable Anniversary were all played as a quick test - all fine and saves read.
All in all I have nothing really to complain about - It could have been a nicer experience (for the general user) if you didn't have to go through all the registries etc. to change their stalking habits but for the sake of acquainting myself with the system it was still rather enjoyable. The overall feeling I have from using it the past couple of days is "I think I like it" rather confusing but it is a better result than the projectile vomit worthy release of Windows 8. At the very least if you are on 8.1 - definitely upgrade, for Windows 7 users I would also suggest it but I would suggest you also pick up Startisback just to keep the feeling more "Desktop".