Sweet's Primer is a helpful supplement to our Old English class. I am currently teaching this class as an elective at a prep-school in Irving, TX. Sweet is brief in his explanations (the entire grammar section is less than 50 pages, I think). And somehow, he makes it seem more simple for the students than Baker, or Mitchell and Robinson. This brevity is helpful because my students, who have already taken several years of Latin, get overwhelmed by all of OE declensions and sound mutations, etc.
Do keep in mind that the purpose of our course is to learn enough grammar to read selected texts and Beowulf. It is only a two-semester course, so we don't spend a lot of time going into the deeper academic and phonological interests of OE.
As a teacher, the main thing I find lacking in many of the OE primers is student exercises at the end of each chapter. Many times, I end up making my own quizzes and homework exercises to help the students become more familiar with the concepts. However, Baker's online workout room is fantastic for this.
Sweet's Primer is the classic, and you can't beat the price.