I've been using the Pentax system for the past three years, and in all that time have been somewhat painfully aware of its shortcomings in the autofocus speed department. However, the Pentax advantages (weather sealing, in-body stabilization, lower cost) have been quite compelling compared to Canon or Nikon.
I've been able to buy (used) a Canon EOS 40D body, and the 100mm f/2.8 Macro Ultrasonic lens for very good prices in the past few days, though. And using them has confirmed that Canon autofocus is at least a generation ahead of the K20D. It's been said that the K-5 has much, much better AF than the K20D, but a K-5 would cost 50% more than I spent for both the Canon body and lens.
So here I am with my new (old) very small Canon system. Obviously, I can't afford to keep duplicate lenses, so I'll probably sell off my FA 50mm f/1.4 and D-FA 100mm f/2.8 Macro, and replace both with the DA 35mm f/2.8 Limited Macro. Of course nothing beats the f/1.4 aperture of the FA 50, but one can't have everything. And the Canon 50mm f/1.8 II is quite cheap. The DA 35mm is still a 1:1 macro (albeit with a very short MFD) and can be used as a general-purpose lens as well.
I still plan to keep the K20D for now, as I can't afford a weather-sealed Canon body. Maybe I'll replace my 16-45mm f/4 ED AL standard zoom with the 18-55mm f/3.5-5.6 WR kit lens (which is weather-resistant! and cheap!) or, get the DA 21mm Limited.
It seems obvious that Pentax strengths are in weather resistance, in-body stabilization, and Limited lenses. Outside these areas, Canon and Nikon have more coverage.