The Pentax is far and away the best value for money, if you can stretch to the Kr you get the new Sony sensor that outperforms any other APS sized sensor on the market (whatever the price). Better low light performance, lower noise (on a par with a Nikon D3, a full frame camera costing thousands of £'s more), wider dynamic range than even full frame sensor cameras, along with the top end K5 it really is a breakthrough camera.
Pentax has the longest list of compatible lenses, 230 'K' mount lenses and that's just the ones made by Pentax. third party lenses would more than double this figure. All will be image stabilised as it's built into the camera, any that are auto focus will work as the motors built into the camera too, all at no extra cost.
Here's a list and genuine user reviews.
http://www.pentaxforums.com/lensreviews/
It doesn't stop there, Pentax cameras, even at entry level, have ALL the functions you will ever need, some only available on a Pentax. Nikon cameras are very poor at entry level, most don't even have auto bracketing (required for HDR) a pretty basic requirement. Canon may have them, but they are buried in menus (3 different menu settings required to set up your camera for HDR).
Pentax are far and away the most cost effective system to buy into.
Nikon cripple their entry level cameras, if you want all the functions you have to buy the more expensive models or, if you want auto focus and image stabilisation the more expensive lenses.
With Canon your buying old technology, things have moved on dramatically, particularly in the last year with sensors. They are definitely more 'clunky' to use than any other manufacturer.
Pentax are really well thought out, which makes them quick to use, have all the functions, the best backwards compatibility of any manufacturer which means loads of cheaply available high quality lenses are available, and the best sensors (which is the important bit) by far.
Chris