I have a K5 and K10D...both grips, both with extra battery in the grip. I have a Km, no grip available and my large hands overwhelm the camera...I like the addition of a grip as I can get a good 'grip' on the camera body with my XL hands.
I bought a K-1, tried the camera for awhile without grip and within 2 weeks ordered a grip and extra battery to go with the K-1. In my case I need the extra surface area that a grip provides and I also like the extra charged battery ready to go, when I run out of juice on a shoot.
I have the 28-105 on the K-1 mostly.
My K10D, K5 and K-1 are all big and heavy mit grip, but so am I
...so I don't notice the extra weight and size....but I do notice the extra security in holding a gripped K series digital body.