I was quite pleased with the Pottermore experience. I waited a few days since I was traveling at the time they were first released so the website wasn't slow at all. I really liked that I was able to link a single purchase to both my Kindle and my Sony accounts so that I can download them in multiple formats. I was on a business trip and away from my home computer so it was nice to be able to make the purchases via a website, wirelessly download to my T1 through the Sony Reader store and then also have them on my iPhone in the Kindle app. Book #1 also made the plane ride home more entertaining!
I don't think the prices are too high. I was expecting them to be in the $9.99 - 14.99 range since I think they are probably in high demand.
I am still a bit wary of the watermarking concept. I guess I just have extreme paranoia that someone could steal your ereader and then upload your books to dark places and you get blamed for it. It seems there would be no innocent until proven guilty in this type of case.