Just got my Goodsmile Co. Tachikoma today. Here are a couple of size comparison shots:
Back row, left to right: GSC gokin, Fewture gokin, Megahouse Perfect Piece, Bandai gokin, Wave W.H.A.M.
Front row, left to right: Hobby Japan Tachikoma File, Megahouse, Organic, Kotobukiya 1/72, Sega mini figure, Movic, keyring
Overall the GSC Tachikoma is pretty nice. It's very heavy (about the same as the Fewture version, but much better articulated). It has a good amount of detail too. About the same number of gimmicks as the Perfect Piece, just different ones. And of course the Major figure is a big bonus. MUCH better than the figure that came with the Perfect Piece (who I forgot to include in the photo). If the price was any cheaper I'd say it was a no-brainer.
P.S. Sorry my camera sux.