Where is the build up?
If it's around taps I saw a great tip on 'How clean is your house'. Soak some kind of cloth in vinegar, wrap around taps and secure, leave for a while and it should come off.
I've just started cleaning my shower screen with a mixture of vinegar and bicarb soda. Mix enough to form a paste, wipe on, leave for a few minutes and rinse off. No scrubbing! Gets all the really caked on build up. I do find that it leaves a bit of powdery residue when dry which I wipe off with Windex. Much easier than scrubbing though.