Pros: Durability, price, amplification
I bought some Beyer DT770 250ohm's that I knew I would need an amp for. Gary was extremely helpful and answered all my questions concerning the headphone working with the amp and which batteries I should get. He also enjoys the photos I send him of my amp with the headphones. I try to take some good shots :).
The length the batteries last, of at least 100 hours, I've never run out of battery life on amp and I've had the same rechargeable batteries in it since I bought it. Sure I've charged them but its a pretty infrequent thing. An A+ amp to be sure compared to the Filo E7 since that is only 80 hours and I am not sure if that battery is replaceable or not. Gary's is :).