I'm on the BOD of an association that is currently using Wild Apricot to do our billing only. We are not using it as our website but we are considering that in the future. I was able to set up the billing and hook it into our existing website, which is WordPress.
THe customization was a little quirky, but I was able to figure out the templates and widgets. It only took me a couple of hours to totally revise the two or three pages we needed to set up our billing and member benefits pages.