I agree, and I felt the same way when I read the whole book. It was hard for me to learn anything about writing from someone whose own style is so flabby.
But the substance of his advice is good, and anyone who enjoys the style would probably benefit from reading it. It's just kind of ironic that he's unable to apply his own instructions.
According to my recollection, there's less of the nuts-&-bolts-of-grammar and more about substance. E.g. Zinsser spends a lot of time arguing that you have to have a deep understanding of a subject, and be able to think well about it, to write well about it. Which ... sure, no doubt. But, um, not that helpful? (At least not to me at the time.)
It's been a long time, though, and the book apparently has many fans on this board who surely know it much better than I do. But no, less technical and more discursive than Strunk & White.
But the substance of his advice is good, and anyone who enjoys the style would probably benefit from reading it. It's just kind of ironic that he's unable to apply his own instructions.