well...
It's a nice program, but wasn't for me.
I am a freelance Web designer who normally uses Photoshop Elements--it has enough of the features of the full version of Photoshop to suit my needs, and is a fraction of the price of the full version. I tried Pixelmator after my Leopard upgrade that prevented my ancient version of PSE (2.0) from working. After switching to a Mac, I didn't want to upgrade my version of PSE until the new universal binary version came out.
Unfortunately, I found Pixelmator lacking key features that I use all the time. For Web graphics, I need a lot more control over text, selections, and layers than Pixelmator offers. I also missed other PSE features, such as the "Save for the Web" feature that has a live preview of the effects of compression.
Right now I am doing a 30 day free trial of Photoshop CS3 to help bridge the gap. After that I will have to use my old PC for Photoshop work until the new Photoshop Elements is released.
If your needs are similar to mine, consider the new version of Photoshop Elements that comes out in March (see
http://www.adobe.com/products/photoshopelmac/). It's a little more expensive than Pixelmator, but it's still under $100. There will probably be a free trial of that, also.