I ran the Early access builds for a few weeks last month. There where some minor issues and stability problems for me but the guys I know who are still running it report the currently available builds to pretty good.
In terms of game performance, I saw equal or better performance in all the titles I ran while I had it installed. It's not like jumping from XP to Vista was back in the day where you would expect to lose performance on some system configurations. In terms of gaming generally W10 will give you Directx12 which is compatible with any card that can DirectX11. Some of the more advanced features are only available in fully DX12 compliant hardware but the point here is that it won't cost you new hardware to get some of the benefits of the new libraries.
The main differences day-to-day are some re-arrangement of menus and a new start menu which is a kind of hybrid of the classic W7 start menu and the Windows 8 mobile/tablet friendly menu. In the last build I tried I think they hit a reasonable balance with that. If you've run a PC in the last few years with W7 or W8 then I don't feel it would take much to find your feet in W10.
In terms of windows 7 for longer-term use, while the free support and sales life of it have now officially ended, security updates for W7 are listed as being supported until 2020 on the Windows lifecycle page so there's no reason you can't run it that long however as time moves on you may find support from device manufacturers and game developers may cease to be a priority, though I wouldn't expect that soon.
You don't need to rush to take advantage of the free upgrade to Windows 10 either. It is available for the first year of release so you can do the free upgrade any time up to 28th July 2016. That gives you some time to let the dust settle a little, let them work out any issues with and and then do some reading and make a decision about whether to upgrade maybe 6 months down the line when there's more information available to you.
