Tiling is an absolute pain, I did one shower enclosure from new, built the stud walls etc and fitted the grosfillex, the other was an already tiled enclosure, I took off the tiles but I'd say it could have been done over them no problem with the right adhesive. It was a small shower so I didn't want to make it smaller by putting over tiles.
No evidence of mould or anything although as usual where you use sealant around the edges etc it does discolour a bit but no grout to worry about! It's quite easy to use once you get into the swing of it and now comes in some lovely colours/finishes, I went for fairly basic white/grey marble stuff at the time as there wasn't as much of a selection. I used it recently again for a ceiling actually and there is now huge choice.