I have used Muscle Milk for a couple of years. I primarily use Vanilla Creme powder. I have also used the Muscle Milk RTD's - especially when traveling.
Muscle Milk has worked well for me, though it is a heavy drink. It has a blend of whey (including the harder to digest and less bio-available whey concentrate) and casein proteins.
I am not currently using Muscle Milk. I have returned to Nature's Best IsoPure. The IsoPure is easier on my stomach, though it is more expensive. The IsoPure is all whey isolate, though not micro/cross filtered - it is ion exchange whey isolate. I recently used an IsoPure RTD when traveling. Again, it is more expensive than the Muscle Milk RTD, but IMO tasted a lot better and was easier on my stomach.
I called Nature's Best and they stated that all of the IsoPure products were gluten free.
The IsoPure Mass was what I used to really jump start my weight gain a few years ago.