You can upgrade immediately by making a Windows 10 installation disc/USB drive (
here) and running it on Windows 7/8. You can theoretically make a fresh installation of Windows 10 too but it didn't like my Windows 7 serial for some reason.
I'm not terribly impressed with Windows 10 so far. I think the interface is worse than Windows 7, and it's not nearly as customizable. It's also utterly rife with Microsloth's counter-productive attempts to be user friendly, I guess, by taking away options in favour of the operating system doing whatever it wants, obstructing simple things and using confusing terminology (I still haven't out what 'apps' are supposed to be, for example). I also turned off about a dozen options relating to data collection to 'improve my experience', though it probably still does that, and there are various other fundamentally stupid features infesting it (Wi-Fi Sense is the infamous one).
That's not to say it's unusable. I haven't had any problems with program compatibility so far. Pretty much everything I tried worked (once I got things like UAC, which can no longer reliably be turned off, to stop breaking them), including a couple of semi-esoteric drivers which still haven't been properly updated. I'd be very surprised if any games which worked on Windows 7/8 would have trouble with this one. But if you don't have any interest in the new interface or any of Windows 10's new features (like DirectX 12, which still isn't being used by anything), I don't think there are any reasons to upgrade for now. Unless you're running Windows 8, that is...