So I recently purchased a Pentax DA40mm Limited (the pancake lens) and took it for a spin this weekend. The F2.8 does struggle a bit at ISO200-800 in some dimly lighted indoor rooms, but I mated it with the AF640FGZ Pentax Flash unit to get the best pictures I could. I think boosing the ISO to 1600 would've made for some better low light shots.
That being said, here are some of the better shots I took this weekend while on a trip away:
AF540FGZ bounce flash + Pentax DA40
AF540FGZ bounce flash + Pentax DA40
Pentax DA40, no flash
Pentax DA40, no flash
Pentax DA40, no flash
Accidentally had tungsten white balance on this one, but it created a really cool effect.
My conclusion is that the F2.8 is a great lens, but for dark indoors is best mated with a flash. I have to work on composing my pictures properly as I realize it's not the greatest in these pictures, but hopefully that'll improve over time!