I used to primarily be a Gigabyte man. I think that they were at the top of their game, with their P35 and then P45 motherboards. (P35-DS3R, DS3L, etc. and EP45-UD3P/R, one of the best overclocking 775 boards.)
I'm not so sure about Gigabyte quality nowadays. People complain about "boot loops" with their P67 and especially Z68 mobos.
ASrock, I've used their boards back in the S775 and S939 days, and they made some fairly unique, if under-appreciated boards, crossover boards with both DDR and DDR2, PCI-E and AGP, etc.
But they've been releasing more cutting-edge boards lately.
I just bought their 990FX Extreme4 AM3+ board, primarily because of the price and feature-set. It gave me everything I wanted (8+1 VRMs, front-panel USB3.0, and floppy and IDE, as well as SATA 6G.) All for only $160, which was discounted by Microcenter by $40 for buying it in a combo with a 1045T.