I'm a Pixelmator guy, but I like Acorn very much. People who write it isn't as powerful as Pixelmator just haven't played around with it enough. There are some things Pixelmator does better, and some where Acorn has the edge.
I think it depends how you use the app. For me, Pixelmator is great because I usually work on a 13" screen, so I don't have much screen estate. With Acorn, I am less flexible because of the big main tool palette, and every time I want to use a filter, another Window pops up showing me a preview of the effect, so I can't see the picture behind it (which changes in real time too). However, I have tried Acorn on an iMac, and wow, that is so cool.
The other special thing about Acorn is the very clean, tidy interface where all the tools are like in one toolbox, everything on its place. Some like that, some don't. I'm more of a chaotic guy when it comes to creativity, so I prefer the sometimes messy, cluttered but fluid interface of Pixelmator, but I have friends who like Acorns interface better.
Same with the effects. Unlike Pixelmator, Acorn has layer effects, so you can tweak the effects during your work, and adjust them at any time, non-destructive. There is also the nice (above mentioned) filter window, where you can combine as many filters as you like until you decide how you want it. You can also save that as layer styles. Cool stuff, but depends what kind of guy you are; for me, that ends in a never ending series of tweaking so I never settle on a solution – for me, the limited functionality in Pixelmator is better suited, where I have to decide for a filter and boom! it's there and I can't really change it later. I also like to merge layers and forget about it. This way I put more effort in working precisely, taking decisions.
Well, everyone is different, and there is lots of great functionality in Acorn. Don't believe those "Pixelmator is much better"-comments, that is non-sense. Before you decide for one or the other app, give both a fair chance, do some work with those, so you can find what app covers your workflow the best, what features you really need.
Sorry, it's a long review, but one more thing I have to mention: the developer is very, very responsive, and really nice and understanding, it's like a dream. If you have a good feature request, it will be heard and implemented as soon as possible. You won't see that with Pixelmator (which has albeit the larger community, more tutorials and stuff).
Anyway, have fun testing those apps!