A browser won't fully pass all of the Acid tests if it follows a few modern changes but it can be a good way of seeing general improvements take shape and show how close you're getting to the fiddly compliance bits. It obviously won't cover all of the newer stuff but it still takes quite a bit to cover everything in the tests.
awesomekling (who's on HN, shoutouts if you see this Andreas you're awesome!) used Acid 2 to help push the development of the SerenityOS browser in some coding streams.
Indeed, the Acid tests are in fact immensely useful for bringing up new HTML and CSS implementations!
There's still a lot of work to do on the CSS box model before the SerenityOS browser engine can render Acid1/Acid2 fully.
Then we have Acid3 which will require a lot more work on the JS engine and DOM API's. But it's all so much fun that it doesn't matter how much work it takes. :^)
There's no actively maintained port right now, but jcs@ ported it to OpenBSD a while ago. His branch[1] is a couple months behind now, but it wouldn't be terribly difficult to get it working again.
awesomekling (who's on HN, shoutouts if you see this Andreas you're awesome!) used Acid 2 to help push the development of the SerenityOS browser in some coding streams.