As with any other type of critic, the critics who write in the Penguin Guide, as well as AMG are giving their opinion, which may or may not be mine. However, the mere listing of less well-known artists has been invaluable to me, especially when I first became re-acquainted with jazz, about seven years ago now. If I hadn't browsed the lists, and read about the artists, I would have had no idea where to start. That, combined with recs from more experienced jazzers is the base of my collection of jazz.
Of course, the Penguin guide lists only CD's, so artists whose work is mainly found on vinyl are a little shortchanged, but that doesn't happen as often now that re-mastered CD's are to be had of the really old stuff.
I haunt a vintage vinyl shop here and am more often delighted, than disappointed in discs I have bought, solely on the strength of the covers and the extensive descriptions on them. The field is enormous and any help is good help, I think.