I am a Pentax K-30 (White) owner who came from the Canon world. I owned a Canon XSi then upgraded to the 7D.
I moved over because I wanted to try something different and I'll be honest I'm glad I did. The Pentax platform has been very good and I like the weather-sealed option for the cost. I know that Nikon and Canon (and now even Sony) get most of the play in the business, but Pentax easily competes with them. Anyone who use Pentax out there, tend to stay with Pentax. They are solidly built and offer a lot of bang for your buck.
I will say that if you want to shoot alot of video, this probably isn't the camera I would recommend. Although it does video it isn't what it does super well.
I'm a big fan of my K-30 and have considered upgrading to the Pentax K-3, but there are a few features I wish that Pentax would work into future designs...like an articulating screen...even if its like the tilting Sony style. This would serious aid in low or high composition shooting.
If you make the switch, just make sure you pick up the WR lenses. No point in having a weather sealed body without weather sealed lenses. 18-55WR, 18-135WR, or the 50-200WR are pretty common to find used at a reasonable cost. (I believe that there are some new releasing lenses with red rings (HD Series) that are also WR)