So your mage wants to make a ring of unlimited teleportation? That warrior, he wants a sword of kill everything +5? The priest wants an ankh of turn everything? You know you can't let your players have anything like this because it would ruin your campaign, so here are some things to keep in mind when letting your players make or have crafted magical items.
1) Is it appropriate for their level? Obviously if they are making it they have to have the spells to even make the darn thing, but you can always goof and let them buy something above their level. Check similar spells and items appropriate for their level to see if it about the same power.
2) Is it appropriate for the character? The mage should not have a sword of dragon slaying. They could make one, if the have the right spells and ingredients, for a fellow player's warrior, but they should not be getting one for themselves.
3) How powerful is it? Artifacts should definitely be handled with care because they should be powerful, but think what a sword of dragon slaying would do to your campaign if your main bad guy is a dragon? Balance is essential.
4) Now that you've deemed it appropriate how much is it going to cost them? If it is truly powerful, but alright for their level, make the components hard to find or ubber expensive. The gathering of components can be several adventures all in itself.