I don't wear glasses but the problem I was having was that when the camera told me the image was in focus, it didn't look focused to me. You can imagine how helpful this is for manual focusing! I had the adjustable diopter all the way to the right (-2.5) but felt like I needed 1-2 more notches. Pentax doesn't seem to have any diopter accessories, so I ended up buying a Nikon one that extends the diopter range to -5.5...and I can finally see everything in focus!
In fact, I just installed it yesterday, took some test shots with the FA 50mm f/1.4, and found that my manual focussing was sharper than the K10Ds AF! Woohoo!
[SHAMLESS PLUG ALERT]
But on the road to this, I accidently bought the wrong diopter add-on before I bought the second, correct one. If anyone out there has the diopter adjuster set all the way to the left (+1.5) and feels they need to go a bit higher in order to see a focussed viewfinder, send me a PM if you would like to purchase my spare diopter.