Pixelmator is a nice clean photo editing program available to mac users. It has a simple and visually appealing interface that is straight forward and nicely polished. The tools it offers cover most of what I would use including a nice spot healing tool as well as all the usual offerings.
The program worked consistently well and I have yet to have it hang up on me and would be reasonably gentle on older machines, load time is certainly faster then Adobe PS though it does slow down as the images get more complicated.
Integration into other programs wasn't too great with not automatically sending files back to Lightroom after saved, I didn't try it on other photo management programs though my quick google search came up with the same problem occurring in Apple's Aperture program.
One big downside is the limit to 8bit images which is the biggest deal killer for myself.
For the price ($15aus) this would make an excellent affordable photo editing program to compliment iPhoto. I would also most likely choose this over Adobe PS elements especially as it is a quarter of the price.