I haven't tried PaperBack myself because it requires Pinboard, and that service costs $10 to sign up. But I have frequently heard good things about Pinboard, with many calling it a great replacement for Delicious.
As you can see from the screenshot above, PaperBack is as clean as Instapaper. And thanks to the fact it's based on Pinboard you'll also be able to use that service's search functions when you need to go find a long lost article.
Paperback is free, but there is also a paid option which adds font choice, keyboard shortcuts, tags, and the option to have Paperback track your reading position in each article. This will let you continue an article right where you left off, which could be really useful for longer articles. The paid option is $5 a month, which is a little steep for such a basic service.