I've been using Pixelmator for about a year now, mainly for web application GUI design. I really like it, but it does have it's shortcomings.
I quite often find myself needing to create primitive shapes like rounded rectangles. The fact that Pixelmator is not a full Photoshop fledged application is
not a good reason for it not to support primitive shapes. Paint Shop Pro from the 90's supported this, for crying out loud!
I also work down on the pixel-by-pixel level at times. Even here Pixelmator fails when it comes to precision.
The inability to drop shadows without a 3rd party plugin also seems odd, considering all the other filters it comes with.
When a photo editor is named "Pixelmator", it creates some expectations that it in my case fails to meet.
I'm in the process of trying out Acorn, as suggested by psychofreak, and so far I'm very content
You can draw primitive shapes and later on adjust their properties such as color, size, line stroke, shadow, etc.