Those welsh roads are great. My folks live up there so i drive that way regularly. Don't try it in the summer time though as there are too many tourists trundling along looking at the view. One of my best drives recently was actually christmas morning - had the whole country to myself it seemed
You have to watch out for:
1) Sheep. Especially early morning.. they are spectacularly stupid and if they've escaped from a field overnight will wander out in the road - even if you hoot or make a lot of noise. No self preservation instict at all! They will make a very nasty mess of the front of your car if you hit one.
2) Police. Richard Brunstrom is chief of police for north wales and he's the most veciforous anti-speeding plod there is right now. His officers are well behind the mood of the rest of the nation and think it is perfectly acceptable to sit behind hedges with radar guns. Helloooo guys! I thought it was all about getting people to slow down, not just about issuing fines
That said, the roads are super. In addition to the above check out the A494 past bala lake and the A458/A470 from llanfair caereinion to dolgellau.