The lack of layers to me would be a significant factor in determining whether pixelmator is a suitable replacement.
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.
Not that anyone is saying Pixelmator is as good as Photoshop. It clearly isn't, but it fills a very important gap in the market, one that on Windows is filled by the likes of PaintShop Pro. A cheaper PS alternative that does everything the casual user needs.
HDR Images: PS has a built in function to take 3 images of the same subject having 3 different exposure levels and will turn them into an HDR image.... usually you shoot one normal, one at +2 stops and one at -2 stops. HDR or High Dynamic Range gives you those photos that look almost surreal. You can really see it in clouds and water! Google it if you have not seen what HDR images look like!