While I love Instapaper for bringing me back to reading long-form articles, I personally moved to Readability a while back. It's $5 (minimum)/month, but it has a lot more polish, and I actually prefer the mobile app to Instapaper's iOS ones - it's nice knowing if I decide to leave the iOS ecosystem, I'll still have my articles, while Marco has refused to make official Instapaper clients for other mobile OS's (understandable, as he's only one man). I also like a handful of features in Readability's desktop web app that Instapaper lacks - scroll tracking, more options for how it looks, etc.
However, if I get an iPad, I may move back just for Instapaper's iPad app. Looks beautiful.