while i wouldn't exactly call the V67 "muddy" in general, it does have this weird midrange "hashiness" going on. i like the tone of the mic a lot, but it's certainly a "situational" mic more than an "all around" mic.
it would certainly NOT be my first choice on an acoustic guitar.....unless that guitar was full of really nasty high end that needed taming (think: really cheap plywood guitar with brassy strings). on the acoustics i've used it on, there is indeed a bit of a muddiness.
i tend to find a much better use for the V67 on thin, nasally vocals that need a little fattening up.....in otherwords, i essentially agree with chess here. again, depending on the voice, that midrange hashiness might work to your favor or not.
see, when i first got the V67, i really loved the sound of it--and i thought it was an excellent mic--let alone "excellent for the price". and it IS an excellent mic on my voice--in a standalone context.....but when i got to using it in the context of a mix, i found something "wasn't quite right" about it......and then i heard that weird midrange. but it took me hearing it in the midst of a mix before i heard what others have said. still, tonally, i like it on my voice. and on a friend's voice, it worked very nicely in a live acoustic session.
my favorite use for the V67 is as a drum overhead. that weird midrange really helps it bring out the meat in a snare drum, and given that it's not hyped on the high-end like a lot of chinese mics, cymbals aren't piercing on it, either.
i don't have any experience with the 990, but from what i've read around here, it would seem to be the opposite of the V67, tonally speaking.
so does that make me a lover or a hater of the V67? neither, i suppose--it's definitely got its uses, and for some vocals it's quite good. for $100US, it's not a bad mic at all, and even better if you can get one used. there are certainly a LOT worse mics out there.
cheers,
wade
PS--spend the extra $20 and get a shockmount for it