The only real drawback is the size, they are bit chunky compared to the 3rd + 4th Gen ones. Apart from that 2nd Gen. did have some advantages over the 3rd Gen, in that it had the 'classic' touchwheel control with the 4 real buttons round the wheel. The 3rd Gen. had the buttons above the touch-wheel, which was a bit awkward, and they were touch-sensitive and offered no feedback when you pressed them.
4th Gen. are the ones to go for though, with the click-wheel and 50% better battery-life than 2nd/3rd Gen.