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Hi there,
I own both programs, that is Pixelmator 1.5 and PSE 8 (and PSE 6), and I have to admit that I like Pixelmator much more. It seems just easier and more straight forward besides being snappier. In PSE I often feel like taken by the hand and as a consequence ending up with losing some of the control.
Some people rant over Pixelmator but I could never find their point. Most is just hot air of some "designers" being frustated of life. An often recitet critic: the Pixelmator GUI. Personally I don't think that the black HUDs in Pixelmator are harder to read than the tiny semitransparent smart tags in PSE. Both programs come in dark grey anyway and the tool bars are on both readable to me.
Still there are 2 things that bug me in Pixelmator 1.5 under Snow Leopard: First and most obvious since version 1.5 the integration into iPhoto as an external editor is somewhat broken as I can't simply press "save" and it saves back (worked in 1.4 and the developers are working on a fix, which will be available as a free update). I have to "export to iPhoto" or save to desktop and reimport into iPhoto. I don't know about Aperture though. Second the automator actions are cool, but only 32 bit, so I would have to restart Automator in 32 bit mode to use them (not a big deal, I don't use that very often).
Of PSE 6 and 8 I consider the problems more serious. First of all, it is very uncertain, what the plans of Adobe for OS X support are given the fact, that they dropped PSE 7 and silently dropped stuff from version 8 that is available in version 8 for windows and version 6 for mac. The support for coming OS X versions in existing Adobe products is about zero. They expect you to buy their newest product to have support. That's just greedy. Updates don't work on PSE 6 for me. They just cancel with an error. And last their activation process can make you pull your hair out. Just google for it.

To summarize I would buy Pixelmator again but not PSE.
On a side note I would also consider the following 3 programs as competitors. I don't know them in depth though:
Photoline
Pixel
Gimp
Best regards
Coffee-Drinker