I'm bumping this because I got a chance to try it out earlier.
The word "wow" would be an understatement. The Apple in-ears sound SO much better with Fontopia sleeves. I was extemely disappointed with the Apple phones and hadn't been using them; the bass was okay, but they were slightly tinny (cymbals sounded especially bad) and it was hard to get a good seal, especially in my right ear. The Fontopia sleeves correct both of those things. They stay in great, providing much better isolation and they make the headphones sound remarkably better; the "tinniness" is virtually gone. Even better than the Sennheiser PX100s I'd been using.
If you were disappointed with the Apple phones, but can get your hands on some Fontopia sleeves, definitely go for it.