That is entirely subjective. Guys who like them love them. Guys who don't hate them. It's kinda like being at a party and dancing to music, but you have headphones on.
I don't agree with that statement. To begin with, I doubt anyone has research data to prove that liking is loving and not-liking is hating.
The opinions on in-ear monitors that I have come across in my years of running sound vary. On the whole, those that have tried them see the benefits, which are significant. I'll augment my earlier comment with real-world experience, which is obviously subjective, like everything. Take it or leave it, but it is based in actual user experience, which is what this forum is about.
To start, fills to the mikes are reduced, which in turn reduces feedback potential. The FOH mix becomes more manageable, as spillover from floor monitors is reduced or limited. Stage volume is reduced, as floor monitors generally move the entire volume up, so that things can be heard through them. As volumes increase, the stage mix requires a higher volume to discern nuances, which in turn causes everyone else to turn up and you get the vicious circle of everyone chasing the volume, which I'm sure every musician experiences. In-ear monitors help reduce or even eliminate this.
Perhaps most importantly, hearing damage can be reduced (this depends upon the type of in-ear monitor one uses). A usual stage mix on almost any live (rock) show often reaches levels in excess of 110dB SPL. Hearing damage occurs at 85dB. The threshold of pain is about 135dB, which is about the SPL of a bass drum miked at 3 inches. Even wearing simple tuned earplugs like those from Hearos helps reduced SPL to around 50dB-70dB. The point here is that if one is not wearing hearing protection when one is playing live shows, one is at extreme risk of permanent hearing damage. Wearing an in-ear monitor can protect (to a degree) your hearing and feed you a cleaner signal at lower volumes. This, in my view, is a huge benefit of in-ear monitors. You are actually hearing the music
better than with floor monitors.
Like anything, there are frustrations, and some cheaper technology doesn't work well, which negatively biases some users' experience with them. Like all things, you get what you pay for. Buy decent gear and your experience will generally be good.
So, all that said, I do think they are good and worth the cost.