I played with In-Ear-Monitors (IEMs) for years, just brought a 1x12 and my rack with me. Yes, I could hear myself and everyone else just fine....in fact, a little too good. You really couldn't tell what the ambient sound was like without a couple mics pointed at the crowd or some overheads.
Also, we couldn't talk to each other onstage, all of us had our earphones in...blocking out that ambient noise. So, I started pulling one out and listening that way....eventually, I just left the IEMs in a briefcase and don't use them now.
They make it way easier to sing without blowing out your voice, but again, I can't tell what is going on onstage, feels so separated from the whole experience.