I've got a few cardboard boxes full of VHS tapes, mostly pre-recorded. I was going to sell them but to be honest I found they wouldn't make hardly anything and it wasn't worth it, so I decided to just keep them. I've got a 'Super VHS' player with some clever features on it, so the image is pretty good from most of them considering the age of them. I haven't bothered to replace them all with DVD either. I've probably only got somewhere in the region of 15 to 20 dvd's in all.
Re: the laserdiscs. As said above some are really rare and quite a few laserdiscs change hands for good money. They are still pretty good and I wouldn't mind getting a laserdisc player and some discs for novelty value more than anything. I saw a comparison between LD and DVD a while ago on a website somewhere and in some cases the LD was better. I think that it was mostly in the sound department that it won out.