If you've never used in ear monitors on stage when performing then you're going to be amazed at the difference they make to your performance. Even the cheapest ones work well enough to be worth the money and will give your vocal performance an immediate boost.
I've always said and I continue to say that the best thing any person can do to improve their singing is by knowing how to listen. Listening involves hearing. Using these monitors you are able to hear your voice easily and as a result you will sing better right away because it's so much easier to can hear yourself. The first time I put these things in I used them for a rehearsal with the band I play in and after just a few songs the guys said "you've never sounded better!"
That's not the best part though. There are different styles of earbuds available for these systems and some of them are sound isolating. This helps to reduce the stage volume in your ear but still allow you to hear your voice more than adequately. The end result of this of course is that you are saving your hearing. Not only that but you'll likely find that you're less fatigued at the end of a performance or rehearsal when you use them as well.
The in ear monitor system I bought was about the cheapest ones I could find. Normally I don't buy the cheapest but I was unfamiliar with using them and wanted four receivers so the rest of the band could all have one as well, so I opted to spend less money this time. However I will likely replace it with a better system in the next year or two. It certainly has more than proven it's worth to me!
If you've ever wondered how you can hear better onstage (without killing your ears) and be able to improve your vocal performance as well then this is absolutely the way to do it! Now that I have tried them I will never turn back to using floor wedges again.