They're like the proverbial curate's egg...good in parts. Some of the titles are excellent, while others a not so great. It all depends on the author. They are well illustrated but I'm not sure there's anything particularly startling about the photos they use in many of the volumes. Certainly many have been published before. Similarly, the text will not win any awards for uncovering new or deep/meaningful insights about the aircraft type covered.
My recommendation is to be selective and check whether the Crowood book adds anything to your existing library. If it does provide new material, then go ahead and get it. If not, leave it in the bookshop.