Damage - It's not that good by itself, but since he'll probably be dealing critical hits every blow, it will be good.
Critical - Guy has the highest critical rating in the game. He can get all the way up to 94 with a Wo Dao.
Speed - He'll be hitting twice unless you give him an exceptionally heavy weapon. Which is why you DO NOT give him an exceptionally heavy weapon.
Dodge - He averages with 81 before support conversations, which is very high. This high dodge compensates for his low defense and resistence. After all, if the enemy can't hit you, does it matter how much damage they COULD do if they did?
Growth Rates
HP - 75%
Strength - 30%
Skill - 50%
Speed - 70%
Defense - 15%
Resistence - 25%
Luck - 45%
Average Level 20 Swordmaster Guy
HP - 53
Strength - 19
Skill - 28
Speed - 30
Defense - 12
Resistence - 10
Luck - 21
Total - 173
Supports
Rath - 21/20/20 turns
Matthew - 24/27/27 turns
Karel - 26/27/27 turns
Priscilla - 41/40/40 turns
Louise - 81/80 turns
This is how you get his Critical rate up to 94%.
35(Wo Dao) + 15(Swordmaster bonus) + 15(A support) + 10(B Support) + 14(28 Skill) + 5(S in Swords) = 94. Since there are only 2 Wo Dao's in the game, I recommend substituting the Wo Dao with a Killing Edge. While it causes Guy to lose 1 speed, he'll still probably hit twice. It also reduces the critical rate by 5, but 89% is still very respectable and the Killing Edge increases damage by 1.
Rating - 9/10. You have Guy for a VERY long time, he gets a stat bonus in Hector's Hard Mode, and he has higher stats then many units.