The scenery is jaw-drop spectacular, the drops off the edge of the road precipitous, and if you aren't plumetting a few thousand feet to oblivion you are slamming into a cliff, so try and stay on the black bit and it delivers the full 'high pyrenean pass' riding experience. Just one thing: If it is dark or raining, take the tunnel, your sphincter will thank you! Probably not much fun if there is any risk of ice, either, so early or late in the year could be scary.
Assuming you are coming from France - you will probably com via D117/N20. When you hit Hospitalet-Pres-L'Andorre you will turn onto the N22. Somewhere about here look to take a break and do what ever you do to prepare. This is a seriously twisty road - and a fairly fast one - most of the hairpins you can safely take between 60 and 80 KPH but there a quite a few tighter slower ones - the good news is you will evenly wear every surface of your tires - the bad news is some of the corners merit 'back brake dragging' and that's going upwards! Low season (the Pics are from April 19th 2007) the roads are clear of tourists - so we only had about 5KM when people got in the way. The rest of it is indescribably brilliant! We did our base (Montrejeau) to Andorra and back in a day - 480 KM 8-9 hours ride time - every KM briliant! took the Fazer but the MT03 or the ER6 would be better. The route (all here) is D618/N117 and this final bit N22, CG2 CG3) if you're down here - treat yourself! Don't forget to look at the Pics!