My advice, Find some backup system and do a complete image of your system. Then install it but, have a backout plan, just in case you don't like it.
I'm one who is not going to be like everyone else here. First, I love Windows Phone 7/8 and everything about it. I have Windows 8 installed on a partition on my desktop/gaming computer (that I use mainly for games, pretty stripped down) and I have a Samsung Slate 7 with Windows 8 (tablet). My first thoughts about Windows 8, It sucks with a keyboard and mouse and I still think it does (after a few months with it). The Metro interface is really designed for touch and it does not feel fluid with a keyboard and mouse combo. Some will disagree here but, I think Windows 7 is a lot better with a keyboard mouse combo and a lot of tasks are quicker in this aspect. This is why I suggest doing a full backup because you can restore you Widows 7 just like it currently is.
With that being said, with a touch screen, it's pretty darn good. Windows was never really tablet/touch friendly, They made some interfaces that helped but, the Metro interface is great with a touch screen. I really like it on my tablet. Even from using IE full screen to doing simple tasks on the desktop. The app selection is ok but, not as large as some would like.
If you can get a license for free by all means, take it, keep your key(I would never turn down a free key like that) and even if you don't like it, you can go back. Just what ever you do, have a backout plan, just in case...
Good luck !