I use them mostly for vocals, and the resultant recordings are extraordinarily warm and vibrant. Beautiful mic's - especially for the price. M-Audio is certainly attempting to undercut everyone else when it comes to affordable condenser mic's and for my money, they've succeeded!
I'll buy plenty more mic's from them in the future, rest assured. No complaints out of me.
I've also used the Luna's to record a metal band, a prog jazz quartet, a concert for a pianist playing in a big hall on a huge Steinway D, and the local church chorus.
I've also used them to build some really nice, 48khz 24-bit private sample libs of my own.
Non-musically, I've recorded seminars, done a ton of foley recording with them, lots of ADR, and a bajillion sound effects with them (the poor man's walla recording scheme - run my Luna into the XLR accessory shoe on a DV cam, and record straight to miniDV - works wonderfully). My private sound design collection has been built 100% with these mics, and I'm exceptionally pleased and proud of the results.
So they've seen quite a bit of action and have never let me down. I'm always blown away by the quality of the sound from the M-Audio Luna, and I have worked with a decent number of super expensive ridiculous studio condensers in my time to know that these M-Audio mics are abnormalities when it comes to recording hardware these days - affordable yet startlingly effective; a rare combination. Makes me wonder why I don't hear about M-Audio's mics more often, but ah well... it'll be our little secret!