I was an avid Instapaper user on my iPod Touch until I got an Android phone. Marco has publicly stated his reluctance to support Android, and the third party apps for instapaper were all pretty pathetic. So I switched to Read it Later for a while.
Until I got a Kindle, which changed the game for me. Reading on the Kindle is so much a better experience than on another glowing screen that I don't even bother to try to read long form on the phone anymore.
I went back to Instapaper for a while, using a Windows app called Wordcycler to sync, but now use Readability's Send to Kindle bookmarklet. It cuts out the syncing part (the Kindle just downloads content automatically when it has an internet connection), and I've found that it retains author and publication data more reliably.