I currently shoot a Pentax *istD. It performs very well in low light/high iso situations and the DS2 is probably better, as the D is now 2 years old.
I bought the Pentax because at the time its main competition seemed flawed- the Nikon D100 reputedly had "soft" pics (matter of interpretation/understanding, turned out to be untrue) and the Canon 10D had focus problems. Pentax had a sterling reputation for its older film cams and the ergos were good so I went with the D.
While I've been happy with it, I'm less than thrilled with Pentax's strategy (or apparent lack of one) for their dslr line. As others move forward, Pentax seems to move backward (if at all). The DS was a step back from the D in many features and the DL is a step back from that. They killed nearly the entire line of AF lenses and have been pretty slow in replacing them, though the replacements are very worthy.
But, seeing as I'm getting older & not younger every day, I don't feel that I can wait around for Pentax to define an upgrade path from my D to something better. Not to denegrate Canon (because they make fine cameras/lenses) but they've just never appealed to me. The Nikons seem more user friendly/ergonomic to me.
But now Nikon has the D200 coming in December and Pentax is rumored to have a D replacement coming around the first of the year. Will it fill my needs? I don't know, but I have my doubts. If it doesn't I'll be switching to Nikon.
Now, having put you off Pentax, I'll say that it depends on what you want in a dslr. If you don't plan to buy a ton of glass and don't expect the latest & greatest to show up every 12-18 months then it's a viable option. A Pentax, like nearly every other dslr made, will deliver stunning pictures for you. It boils down to what you want to do with it and how much of a "system" you need with/for it.