I've tried this, and while it works very well for isolation it tends to have a negative effect on the performance. Sound quality and claustrophobia are important in making the singer feel more like they're "in the song" - the tone they're hearing is going to affect their performance. It's like trying to record a heavy metal guitar part with a guitar you have to play completely clean and dry because it'll be reamped later, you're not going to get the best performance that way. Also had issues with the level of the earbuds vs the volume of the singer's own voice in his skull.

Monitoring/mixing headphones is a bit of a conflict, since generally speaking open backed will be better for mixing. If you're mixing primarily with monitors, I'd say get some closed back with a flat enough response to check your mixes (yeah the m50s are good enough for a preliminary 'on location' mix). If you're going to be mixing primarily on headphones, I'd recommend the absolute best open backed pair you can get and use a regular old pair of closed headphones to track with. Or something like the cheap Roland cans that are used for e-drum and piano practice.