K5 is a big step ahead of K10 and also NEX and even most of the 'A' series SLTs.
Smaller, more sturdy, lots of fast access buttons, better tweaked sensor, faster AF
I've used it since Oct 2010 moving up from K7 and K100D before it.
Its the camera that I am the most pleased with and the one model that I am very sure can last me for 1-2 more years unless my hands get itchy and I give in to CBA.
I honestly don't foresee that significant a leap in sensor and camera tech in the coming iteration of the K-??
The NEX7, A77 gives a glimpse of what the sensor of the next Pentax model will have (imho, 24mp of the 16mp K5 o/p, so nothing that fantastic)
AF and such may improve, but the K5 is no slouch.
I've shot against the likes of D700; D7K; A700; D90 in a couple of events and the K5 was as good if not the one with more in focused shots.
Small Pentax primes are well suited to be lugged around with the camera in a street shooting environment, without the hassle of large encumbrance of many other systems.
I shoot street and candids every now and then with various lenses.
You can see my Flicker shots here :
Street Photography - a set on Flickr Thaipusam - a set on Flickr