Pentax have never been that popular compared to canon/nikon, but what they offer is something different and unique, they're not trying to beat canon/nikon they're trying to carve their own niche.

If you want to read stats and talk about noise then DP review if where you should go, but if you want to read about how it feels to own a pentax, and why they exist compared to canon/nikon then read on.

I see pentax as the leica of the SLR world, small (the smallest SLR on the market) with very high quality prime lenses.

Pentax cameras have great viewfinders, much bigger than competing canon/nikons of this size and price.

The k5 has a silent shutter- a dampened mirror box that provides a very quiet shooting experience that until the 5d3 has not been emulated by canikon.

In body IS, this is amazing- while not as effective as in lens IS, it means that all your wide angle lenses are stabilised, and even your 50 year old manual focus 50mm is stabilised.

The weather sealing on a pentax is best in class, much better than the canon/nikon equivalent, I trust my pentax to keep on shooting.

Another advantage is availability of great quality, cheap K mount lenses. Every canon camera can take these lenses with an adaptor, but when you use them natively on this pentax you don't have to stop down before the shot is taken!
Nikon has a similar advantage but their old lenses still carry a price premium, old pentax lenses are cheap cheap cheap, where as their newer pro zooms like the 50-135 2.8 are expensive- don't buy pentax to shoot professionally, buy pentax to shoot with old glass, or the wonderful limited primes (expensive but very nice).

The downsides: owning a pentax comes with a price premium (just like a leica), accessories are just rarer and lenses much more costly. Also there is no upgrade path to full frame.

Video mode isn't anywhere near as good with the pentax, it's quite clunky, and feels like a bit of an after thought.

My main gripe with pentax is their menu system, I never really got it, changing AF points takes way too many button presses, and I prefer to shoot the camera like I did with film, with a manual lens, iso 400 and changing shutter speed and apperture to get the correct exposure- the pentax is a camera for photographers, and purveyors or finely crafted lenses.

If you are in the market for an APS-C camera, and you don't fancy full frame, and you don't want/need the complex AF of the 7d, then the k5 has it's own charm and comes highly recommended.
But if you want to do flash photography, or shoot with fast zooms, then maybe canon or nikon is a better option for you.