I've used the OM6 and OM7 quite a bit, more as a singer than an engineer.
I think that if you AB it with an SM58, the 58 sounds more open, more natural. But it is amazing how the Audix mics kill drum bleed, and are great if you get a singer who stays right on mic.
Another Audix drawback is it seems they don't hold up as well over time... I feel like all my OM7s need new capsules, while my Shures sound nearly the same after 10 years.
Right now I'm using Sennheiser 935s, which sound cleaner than SM58s in the high end, and have a tighter low mid.
cheers,
denny