I think part of the appeal is that it's so genuine and traditional, and not just a tourist attraction. It has a very long history - it was mentioned in the "Satzungsbuch" of the city of Munich of 1310 or 1312.
There's a large section with stalls selling antiques (also some antique furniture) and all kinds of interesting old junk. You can easily spend the whole day rummaging around. Also stalls selling crockery, pots and pans, kitchen gadgets, knives, scissors, all kinds of brushes, socks, used
Lederhosen, old musical instruments, wooden toys, hand-crafted jewellery, cheap umbrellas, herbs and spices, baskets, glass marbles, rugs... I've bought all kinds of useful, useless and just plain bizarre things there over the years. Still have a lovely Moroccan leather handbag that I bought on my first visit 32 (OMG!

) years ago. And the lovely sexy Moroccan guy who sold it to me still has his stall there, too. (Well, he was sexy 32 years ago.

) There's also a kiddies' funfair and a Biergarten.
If you're into serious treasure hunting, my advice is to go there when the weather is looking a bit dodgy. If you go on a beautiful sunny day it will be packed and you'll have to push your way through the crowds to look at things.
The Auer Dult is held 3 times a year, always for a week including the weekend before and after. The dates for this year are:
Maidult: 29.4. - 7.5.2006
Jakobidult: 29.7. - 6.8.2006
Kirchweihdult: 14.10. - 22.10.2006
Opening times: Mon - Sun: 9:00 - 20:00 Uhr (except the Kirchweihdult which closes at 19:00.
Location: Mariahilfplatz.
How to get there: Bus 52 or 152 to Schweigerstrasse, or Tram 27 to Mariahilfplatz.
Official
Auer Dult web siteMore info (auf Deutsch)
Lots of nice photos
here and
here