I have just finished the pullover, size 2T, in Valerie Wells' Della Micro fleece and a coordinating solid pink Winter fleece (pictures up in Flickr tonight hopefully). I thought I'd list a few things of note for those who haven't used fleece before, but would like to try with this pattern.
Most important, I think, is that your fleece is light weight. I found that when sewing the side seams together my pullover got bulky around the pouch. I didn't have troubles sewing through these layers (my machine came with a leveler thing that was helpful in keeping my presser foot aligned with the fabric), but it's worth noting. If your machine can't handle this you might be in for some unnecessary frustration. I intended for this to be a layering piece and the light weight fleece is going to work perfectly over short and long sleeves through the fall and winter months.
Another thing to note about fleece is that well, it's not a quilting cotton, and therefore does not hold a press. However, if you've used many of O+S patterns then you have surely come across Liesl's tip for running a basting stitch as a folding line. Fleece is a perfect fabric for taking advantage of this, in my opinion.
An advantage to using fleece, other than its obvious warmth and general coziness, is that it doesn't fray. The seam allowances are perfectly fine if left alone and unfinished. I always trim mine down, but don't feel obligated to overlock them.
The pattern is very friendly toward this fabric (for the record, the Sailboat top is as well) and well suited. The fleece is easy to use and if you're patient you'll get really fun results!
I'd love to hear others' experiences and tips on using fleece.