Pros: Noticeable improvement in some cases, very little improvement in other cases
Cons: Everything is on front panel
So, this amp makes the signal just a bit cleaner. It was pretty noticeable on my Q701s, noticeable for general clarity, separation, esp cymbals got a bit clearer. For my HD 558 (w/ foam mod), slightly noticeable, but not that much. For my closed-back headphones->very minuscule, you might notice if you listen really hard. When I bought this, I just needed power for my Q701s so I was surprised the quality actually improved, thus I might have exaggerated my initial impressions a tiny bit. Overall the difference minor even for the Q701, but still pretty noticeable. It's not OH WOW I CAN'T BELIEVE THIS, IT'S LIKE DAY AND NIGHT kind of difference.
I bought this amp for my Q701s (Which I won't receive until Friday). When it came I took it out just to make sure it works (All my current headphones are low impedance and don't require an amp). I hooked it up with probably one of the worst possible sources you can imagine: a Nintendo DS (I was in the middle of playing a DS game when it arrived). Of course there was volume, but I also almost immediately noticed that the soundstage got bigger and the instrument separation became better. This was on my AD700s. I thought to myself it must be me imagining things. Just to make sure I switched back to directly plugging into the DS and the DS->Objective2->Ad700 setup. There really was an improvement, not just volume, I adjusted the knob so that the volumes were the same. The improvement is definitely real and doesn't require careful listening to notice, it was almost immediately noticeable. Needless to say, I am impressed as I have always been skeptical of headphone amps, but this one made me believe all the hype.