Yes, I've upgraded a Sierra installation on a MBP mid-2009 17" (macbookpro5,2) with a 512GB SSD to High Sierra. For reasons of backwards-compatibility (mac:Office 2011 etc. pp.) I've
not switched from HFS+ to AFPS.
The machine's performance seems comparable to the one under Sierra, it's at least not slower. IMHO, the real disadvantages of an upgrade of the macbookpro5,2 to High Sierra at this point are:
- Hibernate/sleep mode is currently not entirely reliable - the machine will go to sleep most of the time if you close its lid, but it will take ~40 seconds in hibernate mode 3 for that. And going to sleep and wake-up itself are not entirely reliable.
- The wonderful gfxCardStatus tool for switching from discrete to integrated nVidia graphics does not work anymore under High Sierra. At least not on the mid-2009 machines. This leads to a situation where your macbookpro5,2 will always start with discrete graphics activated anytime you do a reboot and you'll actually have to logout (!) for switching to internal graphics (the preferable mode for most tasks as it's generating far less heat and (therefore) fan activity). Which is really annoying. I've contributed issue reports in both remaining GitHub repositories of gfxCardStatus (1,2).
- There seems to be a bug or at least an issue with the iCloud login on macOS 10.13 High Sierra where you can neither logoff nor login with your Apple ID in the relevant dialogs, but will be confronted with a "This action could not be completed. Try again" error message. It's possible but in no way confirmed yet that this is in some way linked to a downgrade of iTunes 12.7 to 12.6.2.
If I had to decide again whether I should upgrade the macbookpro5,2 from Sierra to High Sierra, I'd now rather refrain from it at this point for the reasons above.
Yet I've still to acknowledge that dosdude1 has done a phenomenal job in providing High Sierra patcher - it's a really nice example of an individual extending a completely decently working machine's lifespan, which Apple had artificially reduced, again in a very sustainable manner.