i've not had any problems with stock formula bearings, they lasted me a long time. cheapo replacement bearings ordered from the bike shop, however, lasted less than a year for me. i'd get some phils, or if you don't want to send away pop into your local skate shop and ask for some quality 6000 bearings (inner diameter: 10mm outer: 26mm width 8mm)

shameless plug for my howto thread