The Crack is a very fun and not difficult build. You can find used ones that have already been built on the market, but if you have about 4-6 hours over a weekend or two you learn a TON by building it.
Of the two amps I much preferred the crack, although it only pairs well with high impedance headphones such as the sennheiser HD6-- line or others above 250 impedance. Measures poorly, sounds amazing, so I'd recommend listening to see if it meets your preferences.