My girls liked it at first, but now that they are doing "C" they can't wait for it to be done! Not being one to waste books/money I am making them finish it, and I told them I wouldn't buy higher grades of it. We've been also doing Spelling Power since the beginning of this school year, and they really like it. I like it, too (20 minutes/day!). They only have to study the words they don't know, and that is a big plus with them :).
With SWO they have to read stories (I know, they are short) that others think are interesting. For example, the next lesson is about protecting your skin from the sun's rays! This is not a topic they would choose to read on their own! Not that everything has to be a joy to read, but my girls love to go to the library and choose good books, I don't feel the need to force uninteresting topics on them.
I know that other students really enjoy SWO, it just wasn't a big hit in our school.