I've used Colemak primarily for the past 5 or so years. However, the benefit is primarily in typing comfort and hand strain, not quite as much in speed, although it is probably marginally faster at its limits. I'm still fluent in QWERTY (I'm typing this message with qwerty to make sure it still works :)), so I can switch if needed, or if I'm using an unfamiliar computer (or a phone, etc). Moving to Colemak completely solved the frequent wrist and hand pain I got while typing using QWERTY. Others have had the same experience.