I started with the Moldvay set and was getting into AD&D by the time the Mentzer set came out - I have the books, went up against Bargle, but never used them beyond the introductory adventure. The idea of BECMI was good, but I've never much run a campaign past 12th, so I never got to use the higher level rule sets.
I think the Moldvay set is a better reference book, but the Mentzer is a better teaching set.
Anyone here pick up the later D&D sets - the big boxed "Classic" set (with Goblin Tower, Dragon Lair, etc. add-ons) and/or the boxed "Classic" D&D set with the CD? I have those as well, and tried to use the big boxed set, but I found it unwieldy.