I imagine being a faster lense and higher quality it will definately be extremely noticeable taking photos of people in doors. The difference between the FA 1.4 and the kit lense is extremely noticeable when shooting people and indoors.
Out doors the kit lense does a great job, which is the main reason why I have not purchased a wide angle lense to add to my LBA. I use it for the 18mm, then anything else I would use the FA 1.4, then so on.
I took this with the kit lense the day I got the k10d in the mail (UPS comes at 6sh so it was already dark out). I did not have a tripod so I just left it on a old wooden bench to help avoid the shake for the long exposure. I think the kit lense performed very well.
18mm, 20 sec, f/8 (just crop on jpg right out of camera)