I actually don't and I didn't realise it until today. I don't really use that tool much because I normally just scroll back up. I don't think this is an essential tool but may be good to have as some people may like it.
We have a button that appears as you're scrolling that takes you to the top or bottom of the page. Just click the up arrow to go up and the down arrow to go down. 😛
I don't usually implement them personally. But if a client wanted one I would do it. Although I've worked on many different sites and barely been asked to add a button as most themes these days have them... well most decent wordpress themes anyway.
Yes, I always use a back to top button--globally--on any website or forum that I have. I think it's important for members and users to not have to scroll up to the top after scrolling all the way down the page.
My website has it, even that I don't use it or other members. and that it's there as an option. I had one member use it all the time for the fun side. it bounces
I personally don't, considering I usually use the home button on the keyboard to get to the top of the page quickly, when I don't want to waste time getting to the top.
I didn't care much for it but when browsing with my mobile I find it useful, specially if it is one of those fade but remain within your space area kind of link. I think my forums include a link but it is static.
I usually find these buttons to be a bit disconcerting, particularly when they're always visible (fixed to the screen when scrolling). On my macbook, it's just as easy to scroll to the top using a couple strong swipes of the fingers on the track pad.
From a design standpoint, if you're going to give the user an option to go back to the top, I'd recommend instead just repeating whichever buttons they would be looking for at the bottom. If it's a site and you want the user to go back to the top so they can register, just put another register button at the bottom. If it's a forum, consider putting a "Back to Forum" button at the bottom instead.