I have heard it said that the Old School Primer is mainly aimed at 3+ ed D&D, FWIW I’m also dubious of its utility.
However I have had a lot of fun with OSR stuff of late. Dungeon World (not OSR!) was my gateway drug (despite strongly preferring AW to DW), Luke’s B/X stories and some of Eero’s posts at story games got me really looking.
I mainly played B/X or Lab Lord and flirted with Lamentations but I do think that DCC is very functional on a rules basis rather than just bluster. There are many rules tweaks that increase playability and generate story/plot/events during the course of play and thus ease the GM’s workload. The modules are often fun/weird but so far have tended too far (for me) towards a list of things to hit. Despite this I have gravitated back towards B/X just because it is less rules. Brendan S writes smart stuff too about streamlined/effective procedures.
Two nice things, for me, about OSR are the DIY ethos of many proponents and the focus on making things that are effective in play. This contrasts, for me, with much design-oriented discussion that seems isolated from play.
Having said all this I don’t think that I could have even understood how to use OSR sandbox-style procedures without having played/read AW. This is despite cutting my teeth on AD&D 1E in the early 80s and moving on to lots of other games before 2E arrived.