I've been using the Crown CM-311A for the past few years and it has consistently provided superb vocals. Prior to the Crown I used the Audio Technica PRO8HEx, which was OK, but nothing to rave about.
I tried the Countryman E6I, which is nearly invisible, but regardless of loads of tuning and tweaking, the mic sounded thin and lacked fullness of sound. Beause it was omni-directional, feedback was a huge problem, especially when if you have a soft voice and need a fair amount of gain to punch out a song. I sent it back. And, when I talked with the dealer he said he did not recommend it for singers, but it worked great for speakers and church ministers.
I will be trying out a Shure Beta (can't remember the model), a mic that a friend is current using and says it sounds better than his E6I but does not have the feedback problems. I'll let you know in a few weeks if it fills the bill for me.
The only complaint I have about the Crown CM-311A is it's fit and appearance. It's not a comfortable mic, and it does not adjust well to fit over the ears in comfort. The windscreen and mic head are a bit larger than I would like and because the entire mic is black it sticks out like a sore thumb. I just purchased anohter spare and I think I'll try painting the mic boom and windscreen flesh colored to solve at least one of the problems. Overall, IMO, it's the best sounding headset mic I've come across thus far.
------------------ Travlin' Easy