Pros: Incredible value, great soundstage presentation, ability to adjust sound via tube rolling, get to build it and customise it for yourself
Cons: Can't use it straight from the box :-), may take some tinkering to get the sound free from induced noise, performs best with high impedance cans
The Crack kit is easy to build and sounds incredible. Don't be scared of the DIY approach even if you're a beginner - it's fun and SO worth it!
Mine had some induced noise at first so I replaced the braided input cable with some shielded coaxial cable and it fixed it perfectly. I see that and potentially damaged/faulty tubes as being about the only real possible issue with this amp.
Once built (takes a day or 2 depending on how long you spend in one sitting) the final result is an amp with incredible staging and layering for the price combined with beautifully smooth and cohesive sound presentation. There's plenty of detail, but it's all cohesive and realistic - it doesn't pull apart your music into individual pieces.
This amp easily outperforms significantly more expensive units and is hard to beat with >250 ohm cans like the top end Beyers and Sennheisers. It sounds so sweet with my HD650s.