Caution: I'm biased, reflected in the fact that I purposefully spent money on the K-m because I thought it a terrific camera. I didn't even own any Pentax lenses when I did it
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My primary DLSR experiences prior to the K-m were with Canons (notably the XTi). I had also used the K100d on one occasion.
The first thing that struck me about the K-m is the white balance performance. I don't mean to disparage the K100d, but my experience with the two made them seem worlds apart on this dimension. Its presets better the Canon as well, in my estimation. It even has a very useful tungsten preset (and this seems to be a bugaboo of much more expensive cameras). It has great auto WB performance. I don't see WB talked about much, but Pentax got it quite right with the K-m. Color performance is just lovely with the right settings.
The other notable is the autofocus performance. I don't feel it suffers relative to the XTi, and most generally agree that the Canons are snappy performers. I know this is a gripe applied to other Pentax models. In general, it snaps to focus, and doesn't hunt much, even in low light.
Very intuitive interface, with the key controls at your fingertips. It's a fun camera to use, with great ergonomics.
Head over to Flickr if you want to see some sample photos.
I have a feeling that I may bump my head on this camera's ceiling if/when I become serious about manual focus lenses; other than that, I'm guessing we'll have a lovely time of it!
dr_s