I came into Shakugan no Shana without much in the way of high hopes. I planned for it to be an "in-between" anime while I waited for the next volume of Code Geass to ship. To my great surprise, however, Shakugan no Shana turned out to be a decent anime.
Let me say this first: I'm a complete sucker for romances. Shakugan no Shana is a strange little show, almost "shounen" but not quite. The romance between Yuuji and Shana is a key element. There is action and fighting, but a distinct lack of named attacks or power-ups (though they are not absent entirely).
My main complaints in regards to the story were these: Yuuji is a relatively weak character, despite clear attempts by the writers to present him as otherwise, and the villains are equally unimpressive. The true star of the show is Shana herself. Her romance with Yuuji is intimately tied up in her character development.
The soundtrack was decent, nothing special. I particularly like the first opening song, Hishoku no Sora by Kawada Mami. I went out of my way to get it on my iPod. In the voice actors department, I found no problem. I watched the dubbed version, so I can't speak for their Japanese equivalents; however, it is typically the English-speakers who perform poorly.
Animation was simple and not particularly entertaining. It seemed fairly simplistic.
One last note: I thought the first episode was awful, particularly the first few minutes. Don't be turned away by the giant demonic baby or the floating head--it gets better from there.
Okay, one more thing...bear with me. For those of you are saying, "Well, why'd you give a 6.5?!" 6.5 is a good score on my mental scale. 5 is "average." You see?
Good luck and good viewing.