I don't have an art style preferrence per se'. I do and have enjoyed the Bruce Timm style. But I am, of course, a fan of the more traditional comic styles. And I even like Anime' styles if it's not too stylized.
I agree with the common voice here, that the quality art work is extremely important to superhero rpg products due to the visual nature of the medium that they are emulating. And I myself am more apt to purchase a product with quality art work.
I think any art style can be good if it's well executed, but I prefer it when the art has a connection with the text (especially rules). Spell Compendium for 3E is a good example of this.
It just makes reading an rpg more pleasurable for me.
Art could really be used a lot more in RPG's to support the text, such as providing a visual example of a rule, but this doesn't seem to happen much.
The graphics that demonstrate the various tactics in 3E dnd did this pretty well, but they weren't really "art".