The claim about the Pentax autofocus being more accurate, comes from the brands different AF methods.

When the AF is activated, it measures how much it must turn the focus ring to achieve focus. Then it turns the focus ring, it is here where the Canon system stops and the Pentax system makes a double-check, it re-calculates and spends extra time on fine adjusting the focus. This is often perceived as "focus hunting", but it is actually fine tuning for better accuracy. You can see this yourself - the Canon system glides into focus, where Pentax often glides to a certain point and then moves the focus ring a little front and backwards until it finally stops. The last part is the re-calculation, the double check, done by the Pentax AF that does not happen with Canon.
This means that the Pentax system allows for less margin, if it can't be dead on it won't lock. But the Canon system allows for a slight focus error, and if you use a small aperture then this focus error is covered by the depth-of-field so it is normally not seen by the user, at least not at smaller apertures.

As for the plastic look - the Canon system applies noise reduction on the image data that comes from the sensor, this is a hardware solution, and this also gives the effect some calls "plastic". Some likes this, some don't. It is a personal thing how you like about it, but it is true that images from the Canon and the Pentax are different.

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Take care
// R
http://www.flickr.com/photos/raphaelmabo