I've been using in-ears on and off for the last six months. I wouldn't recommend the one ear versions because they are really disorientating and very hard to sing from unless you have loud wedgest too which defeats the object... If you can, a better solution is to put a stereo mic (like ones for recording onto minidisk) somewhere pointing at the audience just for your monitors. If you have a coluding engineer this can even be faded down during songs to 'mute' the audience.

It helps with singing because it discourages you from straining your voice, and it certainly helps with mobility (how else could you get hi-fi stereo while standing up on stage?). It does cut you off somewhat however.

As for safety, I don't think you can buy a IEM system that doesn't have a limiter built in in some way. I would never have in-ears anywhere near as loud as I would wedges either.

Brand wise the common ones are Trantec (cheap but decent), Shure (probably the best) or Garwood (the original ones, but very very expensive). I'd recommend www.handheldaudio.co.uk for advice too.