Most of the time, I come down firmly on the side of "old school" in these discussions.
However, IMO clear rules for magic item creation by players were one of the best things about 3E. They solved a problem that had been bugging me for years:
1st and 2nd edition modules had tons of magic items in the treasure hordes. Potions, scrolls, and +1/+2 arms and armor were especially abundant, but let's face it--the most common play style was very generous with minor items.
And yet, the rules for item creation implied that even making a potion was terribly difficult, requiring a high-level wizard or cleric. IIRC, in 1st edition the thresholds were 7th level for potions, 9th level for scrolls, 11th level for charged items (with the Enchant an Item spell), and 15th level for anything else (with the Permanency spell). And in the AD&D DMG, even something like a healing potion required a fragment of a saint's relic!
So there was always a severe disconnect between the abundance of magic in the world and the implied setting.
When 3E came along, and players could manufacture their own healing potions for the cost of a feat and a bit of gold, I was thrilled. I even liked the flavor of paying XP to power items, which many people hate. I also liked that they gave Scribe Scroll to wizards for free, reinforcing their scholarly bias.
Furthermore, I never hesitated as a DM to create magic items that broke the rules. Those rules were for players, mainly. If I have a cool idea, I just make the item; there's no need to price it. And if they players want to make a duplicate, I'll just tell them they can't. The techniques for that particular item have been lost in the mists of time, or are a closely held secret of NPC X. Simple!
Now, I realize that the 3E system has some problems. Wands in particular have too many charges, and are probably too cheap. Staffs are ridiculously expensive, compared to an equivalent number of scrolls. And worst of all is the assumption of equivalency between spells; a wand of Fly is enormously more useful than a wand of Clairvoyance, for reasons already mentioned by the designers. Monte Cook addressed this in Arcana Unearthed by placing magic item cost multipliers, and sometimes prohibitions, directly in the spell descriptions.
But to sum up, on the whole I want my players to be able to invest in magic item creation, at the very least for simple expendable items like potions and scrolls, and preferably for simple permanent items as well. I don't think any rules for DM item creation are needed, except a paragraph reinforcing "make what sounds cool" and some guidelines for what kinds of items might cause problems for your campaign down the line. I think rules for some PC item creation are very useful, and improve the coherence of the game world.