Personally, I feel something in-between pleasure and depression fits well, adding in a cup of envy (being like jealousy, but without the hate), and sprinkled with a dash of inspiration, depending on if it's a comic or not.
God, it sounds like I'm talking about baking a cake of emotions every time I see art XP
I am sometimes jealous. But mostly, I look at the composition and the way of coloring. I learn from it, and it motivates me. My thought overall is 'I can do that in 3 years'! I often look at the bright side.