We use IEMs. They help control FOH volume by limiting stage volume. When the drummer is too loud for another band member, that person inevitably turns up their stage monitor. It doesn't take long at all before the stage monitors are bleeding over into the house and effecting the house mix. Also, IEMs help to protect your hearing by letting you listen at low to moderate volume and still hear clearly.
I guess it depends what you're going for, really. If you want to have a fun jam session where everybody is grooving on everybody else and it's one big party then IEMs might interfere with that since you might feel your in an "electronic bubble" as one person put it.
But if you're sole focus is on performing a professional quality show and you want the whole band to sound good FOH, then IEMs are usually very helpful IME. There are sacrifices you have to make in order to present things perfectly to the audience. You can't be the show and be the audience at the same time. Either you're a performer making entertainment for others, or you're a observer making entertainment for yourself. I'm not saying one is better than the other; they are just different. You have to ask yourself what it is that you are trying to do.
I don't hear the whole band in my IEMs when I play. I just heard what I need to hear to play MY part the best that I can. As a consequence, I miss the show. I don't get to see and feel the whole experience that the audience does. That's ok with me because IMO, that's not what I'm there for. In my mind, I'm there to provide a show to others not myself.
Personally, in my IEM, I have the drums (at a low level since I can feel them anyway), the lead vocal (and sometimes backing vocals at low volume), and the rhythm guitar. That's it for me. I play in an 8 piece (not including vocalists) so I end up missing most of the show but I'm locked in with the drummer and I don't miss the chord changes so I get my job done. I usually miss the saxophone solo, the lead guitar solo, the piano, the synth pads, the harmony vocals, etc. IMO, that's OK with me because I'm not there to enjoy the show; I'm there to perform the show.
There are pros and cons for IEM. I guess you just have to decide what you are trying to do.