Palmer Robertson is always worth reading, and so is this - especially on a book like Joel, where there is so little good stuff available. It reads as a book of sermons rather than a commentary, but it's a good one.
It lost the final star for some odd emphases - e.g. more time telling us what the spirit poured out in the last days doesn't mean, than what it does mean - and that generally slightly erratic feel of sermons that may have been right for a particular place, rather than the core message of the book. Some of the lines of application seemed a little sketchy.
But nonetheless, a helpful little book.