Pixelmator has layers but not layer styles (effects like drop shadow, grad fill, etc)... and it doesn't' have those vector shape tools like rounded rectangles. In designing web sites I use all those features all the time so for me Photoshop CS5 is the clear winner even though it's so darn expensive.
I also love Photoshops powerful Save for Web & Devices feature, history, snapshots, layer comps, transformation tool and other things like that. Plus if I'm sharing a design file in Photoshop the people I work with can open and use it on Mac or PC where Pixelmator isn't as popular and isn't cross platform.