I'd certainly recommend the dedicated thread elsewhere on BC for a range of opinions, advice and information. But my personal experience is as @Phil Starr says. It took me probably 6 months of building up confidence in what I was hearing through the 'phones, getting the mix right and getting the technical side right (finding the right 'phones and buds). I would not go back to physical monitors now for the main band (The Hulla) I play in. I can hear an improvement in my backing vocals now I can hear myself properly, I feel my bass playing has improved now I can hear clearly what's going on and I'm playing with the actual sound I spent ages and ££s getting right rather than a muffled version through old monitor speakers. I would add that in my experience having a dedicated sound engineer has made the process of switching to IEM much easier as I've been able to work with him to refine the mix I'm hearing. The rest of the band, with two exceptions, are not using IEM but I don't think it will be long before the majority see the light.
In bands where I dep, I tend to plug in to the headphone socket on the desk so that I can hear the vocals through my IEM and use different earbuds so that some of the backline sound comes through. It's not ideal but in the circumstances I can get a decent enough balance. In both cases there is a very valuable bonus in that I can manage the volumes and protect my hearing.