Pixelmator is the perfect example of just how good an App can be when it is written specifically for the Mac and thus gets to make full use of OSX technologies such as Cocoa, Core Image/Animation, Quartz, etc.
As opposed to a program written with multi-platform releases in mind, or worse, programs that originated on Windows and were half-heartedly ported over to the Mac.
It's amazing to see how fast Pixelmator performs on my 11.6" MBA. It runs circles around Photoshop.
The realtime filter updates are incredible, even on something intense as a heavy Gaussian Blur operation on a 6000x6000 image.
I bought Pixelmator to run on my MacBook Air after briefly considering installing Photoshop (of which I am a 17 year veteran).
My reasoning had much to do with not wanting an elephant squatting on my 128GB SSD drive. A PS CS5 install comes in at around 1GB, whereas Pixelmator's footprint is about 1/10th that.
Then there was the price. In the UK, Photoshop is about 39 times more expensive now with the new Pixelmator Appstore pricing!
And don't even get me started on Adobe's dodgy price-hike legerdemain outside of the US markets.
While Pixelmator is not as feature rich as PS, it still boasts an impressive arsenal of painting, photo manipulation, and general image editing tools.
I have no doubt that Pixelmator's developers, self proclaimed Mac-fans that they are, and especially now spurred on by increasing sales figures, will keep adding features that Photoshop refugees are asking for.