I had a pair of AV40s. They broke within 9 months.
It's an issue with the QC on the capacitors they use. You can sometimes convince M-Audio to fix it themselves but they just fit the same quality capacitors so it's a crap shoot over whether you are lucky to get a good or bad part all over again. There are user instructions on the web site on how to fix it yourself with a screwdriver,soldering iron, sharp knife and a > $5 part.
M-Audio make and sell a lot of gear so it's hard to say if they get a disproportionate number of returns but I'd say they do. You pays your, to be fair - very little, money and you takes your chance.
I was so delighted with the high quality, desktop monitor paradigm I bought a pair of ADAM A3X. They cost 4 times as much as the AV40s but since the guarantee lasts 5 times as long I reckoned I was quids in even before counting all the other benifits. If I was skint I'd buy an equivalent pair from Fostex.