Pros: Bass-boost, can get loud, good for less sensitive cans, portable,
Cons: Bass-boost cannot be turned off, no power switch, no EMI shield, bad for IEMs (hissing noise), glossy plastic easily scratched
I bought these for my Sennheiser HD 201, after reading around on the forums that it can help with the lack of volume and bass. On the way home I used it with my iPhone 3G and Philips SHE3590. It helped the bass response and clarity of highs and mids, but for every text message that came (my country, the Philippines, is the texting capital of the world), whether on my phone or others, I can hear a loud static noise (sounds like "tut-trut-tut-trut"). If there is an incoming or outgoing call, it's even worse, sometimes incessant. Note to self: NEVER use this near a phone. There's also a slight hissing noise, but it's probably normal for a cheap amp.
Using it with the Sennheiser HD 201 and my Acer AOD270 proved to be a better idea. It did help with the bass, and it allowed me to adjust the frequency graph with Electric-Q to a level where a flat response and lower bass can be heard. The mids became clearer and the highs gained better resolution. The static noise can still be heard in the background, but not defeaning as on the Philips SHE3590. There is also no hissing noise, and overall the headphone, amp and EQ just work together perfectly.
UPDATE: There is a cracking noise distortion that can be head with my HD 201. It also drains batteries pretty quickly, about 2 and a half days of battery life on a AAA alkaline.