I think you failed to mention the much better Auto Iso. From what I heard it beats both Nikon and Canon. What's your experience with it?
To be honest, I'm unfamiliar with the Nikon and Canon Auto ISO functionality and had assumed it was the same as Pentax! I only had a little bit of experience using entry-level Canons and Nikons. I'll give a quick explanation on how it works on Pentax, and perhaps you can inform me on how it works on Canon:
On my Pentax K10D, the user can either select a specific ISO setting (e.g. 100, 200, 400, etc), or set it on Auto ISO. You can also limit the range of Auto ISO by manually controlling the upper and lower limits of the sensor sensitivity. For example, you can limit the range from 100-400 or 200-1600 or whatever. It's pretty handy, especially if you want to stop the camera from going beyond ISO 1600 (my K10D didn't do very well at ISO 1600. Canons are great at ISO 1600.).
In general, the concept of sensor sensitivity is thought through differently (well, on the K10D, K20D and K-7 at least). ISO isn't a fixed variable any more like in the film days where you were stuck with the roll of film you had in the camera. Pentax thinks of sensitivity as a "first-class" exposure variable that can be manipulated just like aperture and shutter speed. With this in mind, it has two additional exposure modes besides Av, Tv and M: TAv (Aperture & Shutter priority) and Sv (sensitivity priority). In Sv mode, you can manipulate the sensitivity, and the camera will automatically control shutter speed and aperture. TAv mode allows you to manipulate shutter & aperture, and the ISO is automatically set by the camera. (I'm assuming TAv mode is similar to "M" mode on Canon and Nikon, with some form of Auto ISO enabled - please correct me if I'm wrong).
Additionally, since ISO is considered a "first class" exposure variable, you can configure your control dials to manipulate ISO directly. In Av mode, I have my front dial configured to change ISO, while the rear dial controls aperture. This allows very quick control of ISO without having to take my eye off the viewfinder. ISO can be controlled with just a finger, instead of going through a menu (like on a Nikon D3000), or a "two-button press" on a Canon Rebel XS or a "press button and spin dial" on a Nikon D90 or Canon 40D.
(Note that dual dial control is only available on the K10D/K20D/K-7. The K-x does not have a 2nd dial, and thus ISO is controlled similarly to the Canon Rebel XS.)
So in general, Pentax tends to handle ISO a bit differently. Direct ISO control doesn't seem to be a bit deal, but if you're alternating between bright and dark conditions, I thought it was very nice ergonomic touch. For my style of shooting, ergonomics vastly trumped AF speed and ISO 1600 performance. This is why I've stuck with Pentax. I'm weird like that.
