It works, fairly well. But I spent about 15 minutes doing one panel, trying to figure out the minor nuances. I kept thinking to myself "I could do a whole comic in about 5 minutes in photoshop" as I adjusted the person's location, arm, speech, etc (and that was just modifying the characters that were already in the scene). In the end it was just not worth it in my mind.
Also, imagine creating something like Randalls "Click and Drag" comic (xkcd.com/1110/) -- how would you create all these custom objects in the editor without changing it into something similar to an HTML5 web-photoshop, at which point you might as well do that?