Few things to add here:
1. The magical effects you can gain? If you're stupidly lucky, they can be permanent (this water tasted very odd). This can either be *great* (invisibility, polycontrol, possibly even haste), nice (can't smell anymore), or horrid (slow, teleportitis, polymorphitis). The thing IMO that makes it worth the risk is that the good magic effects seem to outweigh the bad.
2. Diseases include: Lycanthropy (every so often you'll turn into a werewolf with generally higher stats but no equipment and only biting as a weapon), tapeworms (after a certain number of turns or so you'll start taking internal damage which can only be cured AFAIK by vomiting until the parasite comes out), and leprosy (every once in a while a limb will fall off; note that if this happens and you go to the healer for help that the removed limb in question will still be diseased, such that if you reattach it you'll be a leper again). There may be others, but I'm fairly sure those are it (although teleportitis and polymorphitis might qualify under certain conditions ).