as someone who has upgraded at least like 50 machines spanning 7, 8, and 8.1...
1. if you care much about performance, don't bother upgrading, back up your data and do a clean install. Upgrading will carry over too many settings from your old OS and it just fucks with 10, makes it really laggy at times, just bloated in general.
2. if you don't care about perf, the upgrade process is really simple, and works flawlessly about 75% of the time. 20% of the time you'll need to search the internet after upgrading in order to fix some minor bugs. 5% of the time you'll get a bug where explorer.exe rapidly crashes and relaunches after you log in, it's currently unfixable and happens in safe mode too. revert to your previous OS and try again, all i can really say.
once 10 is configured it's honestly great, and it runs really well on my machine (i5 2500k, GTX 970, 16GB RAM, SSD). multimonitor support is improved over 8.1 in some respects but worse than even 8 in others, it's kind of weird but definitely fixable. search is vastly improved, i used to use launchy but no more. you might as well give it a shot while you still can.