I am a huge Bowie fan, I have all of the RCA/EMI albums on vinyl, but my "classic" Bowie cd collection has been lacking because I'd rather find the Rykodiscs with the bonus tracks instead of the current Virgin's. This set picks a lot of the best songs that aren't on the out of print Rykodisc The Singles 1969-1993 (which I believe is still the definitive Bowie compilation, the albums since should be bought seperately). This by no means a "Definitive" Bowie cd (both of his #1 singles are missing, but haven't we heard Fame and Let's Dance enough?), 4 discs frankly cannot tell the story from the 1969-1993 era... However, if you have this, and also track down the 69-93 on [web site], those 6 discs is as good a summation of Bowie's RCA/EMI/Savage years without actually owning the studio albums.
Also check out his recent stuff. Bowie hasn't been commercially successful since the late 1980's, but I think Heathen and Reality both count among the best albums he's ever recorded bar none. He has definately bounced back after having a few awful years in the 80's