I use this a lot in the practice room. I don't really use in on gigs as I'm usually using IEM's. It's very valuable in the practice room for two main reasons I think:
1) you can hear your sound, colour and tuning very well, and are less reliant on the room acoustics... great if you're in a very dry room!
2) it's great for practicing quiet dynamics... because you can hear what you're doing better