Overall this is a great tool for doing photo manipulation. I use it for compositing images and annotating images for promotional graphics. The interface is definitely a little challenging to get your head around at first and I wish there were more video tutorials for it, but once you get your head around the limits and idiosyncrasies, you can really fly.
Probably the biggest missing thing for me is alpha layers... Without having some kind of alpha mask layer, compositing images is very frustrating. One of the more common things I do is duplicate a layer, mask off part of it them apply filters to one of the layers. This works well for applying an effect to only part of the image. With pixelmater on the iPad the closest I could do to replicating this was copying a section of the page and pasting that as a new layer. This works ok, but you cant adjust which portions of the image are masked after your initial copy/ paste which means if you make a mistake early in the process its very difficult to fix it without repeating a bunch of steps.
Overall, Ive been able to do some impressive stuff, but the problems with layer masks and a few UI issues really hold it back.
5 stars because its the best on the iPad, But its still missing some vital stuff.
mtb_ogre about Pixelmator, v2.2.1