Just got my musicboost through the post. Not massive in volume boost but a bigger, punchier and far more three dimensional sound than the stock headphone socket. Drives my KRK headphones nicely which sound miserable and limp running off the iPhone. Shure 425s are great too, the musicboost doesn't dampen the subdued top of these IEMs at all.
As a battery pack the verdict is not so great. It looks like you get just about one full charge from it, and if you do a bit of listening this sucks up power. It looks like the Texas Instruments dac and burr brown opamp driven headphone stage are a bit power hungry.
However as an upgrade of the built in output it's a worthwhile purchase, the battery booster is a nice extra too.