the 40mm XS was the first prime I bought after purchasing the K-5. I have some older manual lenses but I wanted a prime with AF, and the compact size means it's very convenient to carry and use, especially since it's got a fast AF response when I need it. (I try to stay on manual setting all the time, but in certain conditions it's great to have options.)
I cycle through various lenses, but the 40mm tends to stay on my camera for weeks at a time, it's so versatile and sharp!
The compactness allows me to put my K-5 in a cargo pocket or a jacket pocket, great if I am hiking or walking around a city. It weighs almost nothing, as well, so I can move and respond to opportunities immediately. It's also very low profile, most people do not react to it as the do a to an SLR with some giant lens on it. I spend a day and an evening at Baltimore's Inner Harbor, and it worked for street photography, night shots, and impromptu portraits of my kids.
I can't overstate the convenience of its compact size, I'll get the 21mm version next.
You made a great purchase. I look forward to seeing your photos.