It's fine. It's used by quite a few cyclists. The gradient from Bangor to Llyn Ogwen is reasonably steady. The road is quite twisty in places, so there are places where a cyclist can't be passed, but they aren't that long, and the traffic isn't that heavy. As Canardly says, there will be lots of cars parked (illegally) on the verge once you get up to Llyn Ogwen, but that isn't usually a massive problem. Llyn Ogwen-Capel Curig-Betws y Coed is mostly fast downhill. I prefer the A5 to the minor B roads, simply because the surfacing is better.