Today I bit the bullet and installed Windows 10 for a triple Win 7/10/Linux [Fedora] boot. I installed it to a small, 100G SSD I'd had in a development server that died, so that really sped things up. The cheap ass activation I bought from a dubious supplier (kinguin) surprisingly worked. I used Supergrub bootable cd to get back into Fedora, which threw a kernel panic for the most recent kernel, but did boot ok from an earlier one. From there I reinstalled grub, booted back into win10, installed ubiplay and pointed it at my old FC5 installation. The rest is history.  :-P

It's been booting around pretty flawlessly over the past several days, until a Win 10 automatic update, then the next time I rebooted into Fedora and tried to mount the usual ntfs volumes:

Metadata kept in Windows cache, refused to mount.
Failed to mount '/dev/sda[*]': Operation not permitted
The NTFS partition is in an unsafe state. Please resume and shutdown
Windows fully (no hibernation or fast restarting), or mount the volume
read-only with the 'ro' mount option

Turns out Windows 10 has a 'fast boot' option, which explains why it seemed faster (I had attributed entirely to the SSD), which then fucks up the mounts as above. Turning this 'feature' off fixed it. Also, I disabled Windows 10 automatic updates.

I've never been convinced by Windows Fast Startup, even with a Windows only installation. There's a lot of talk about the improved performance, but actually ALL this means is that (if it's working) your PC will start faster. On older machines this may make some difference, but with a recent device with a fast cpu and SSD, we're talking about a handful of seconds. For me, the main issues are that it isn't 100% compatible with automatic updates. It certainly isn't with some of the 3rd party programs I use, and it isn't even 100% compatible with Windows' own update process (which is absurd). I have my main Windows PC turn off at night after its backup finishes (it used to run 24/7 but the criminal conspiracy to keep power prices absurdly high means I look for savings now) and another machine that runs 24/7 turns it on again in the morning. With Fast Startup I had problems at least once a week, often to do with updates, but sometimes with various servers, eg VNC, not starting properly. Disable Fast Startup and this goes away. As you found, Fast Startup also locks the system drive which royally fucks any multi-boot setups.

The boot isn't noticably faster without. It didn't really fuck up the multiboot per se, though I was unable to mount the ntfs volumes in linux without rebooting into Windows 10 first. What I hate is that when you choose Windows 7 from the win 10 boot menu, it reboots and then loads windows 7, instead of just proceeding to load it. Anyway that it works at all is a minor miracle. So far I'm liking Windows 10 pretty well.