I'm running High Sierra on my early 2009 iMac, having just upgraded from El Capitan, and it has received a significant boost in performance. I was worried about not getting security updates once Mojave was released in September (I'm assuming it will be released then), and so I was going to try and upgrade to Sierra, and someone (I forget who) said I should jump straight to High Sierra instead, because it works better with old hardware than Sierra, and I figured why not?
So I backed everything up, and used a patcher. I know it's not officially supported, so I did this at my own risk, but I can say on my end that everything seems to be working without a hitch. The 8 GB of RAM is likely what's helping that work so well.
There's no way in Hades I'm going to attempt the jump to Mojave, that would really be pushing it, but now I know I can safely keep using my iMac, and receive those security updates, as I save up for a much newer machine in the next year or so.
Now I'm not saying anyone with an early 2009 iMac should do the upgrade, just that for me it seems to have worked out quite well. So far, no bugs. Wireless works, printers work, USB ports work, everything loads just fine. In fact, one of the games I was playing before feels a little smoother this time around, though not by much considering the dismal (by comparison to modern) graphics card I have. Still, I'm rather pleased with how this has turned out. It makes me look forward to when I can have the better hardware to really support the later operating systems.