I watched the first season. I think it's a very competent anime, but it's average as far as I'm concerned. I liked it, though.
If you haven't watched Shakugan no Shanatan, I suggest you watch all three episodes right now. I loved those more than the main series, but you kind of need to watch the main series first =/
^Is Shanatan the little omake where Shana is apparently chibi-sized? I'll have to check them out as well.
EDIT: Just finished Shana Second. I'm hoping for a third season, seeing as this one still left a lot of things open...Too bad a third season won't come out for...another 1-2 years. Eh.