I purchased Pixelmator a long time ago, but have never actually used it. I bought it to do web graphics, but their UI doesn't really speak to me. I've never cared for the floating docks like GIMP uses.
Affinity Designer has allowed me to not miss Photoshop/Illustrator at all. It came out right as I was thinking of buying a Mac and switching to Swift development. Saving the monthly Adobe fees helped me justify the cost of a Mac.
Also, the new Symbols stuff in Affinity Designer makes it useful for doing mockups. I haven't tried the layout constraint stuff they added.
For mockups I think Sketch is still easier/faster, but only barely. And it is kind of limited in what you can do, while Affinity Designer isn't. It's made me want to invest the time into getting good with Affinity Designer, since it's so powerful and fast. The buy once thing is also nice - I think paying monthly for desktop software is mostly bullshit.
Sorry for the long rambling, I just really want to praise Affinity. When I see that there's an update I actually get excited, and they keep adding non-trivial features. It's how I wish other software companies worked. I hope they do well and can continue with their current business model.