I've been using Win 8 for about 9 months now.
I personally don't use the start button so I don't miss it. The one annoyance that I do have is the mouse over pop up menus. When I'm trying to do things often times I mouse over the "pop up area" for the search bar and the windows market apps list, which is slightly annoying when it covers up what I'm trying to click on.
That is my one and only complaint about Win8.
I do like the fast boot option that comes with Win8 if you have a compatible mother board. My system boots up in about 10 seconds.
I wouldn't pay for the upgrade to Win8 though if you already have 7 it's not worth the money. If you're getting it for free it's worth trying out, it only takes about 5 min to install on a SSD.