Everybody knows the Woodstock Music & Art Fair was the biggest, most zeitgeist-defining of 1960s music festivals. But how good was the music, really?

The 40th anniversary offers a chance to listen in more detail than ever before. That's thanks largely to Rhino Records' six-CD boxed set, Woodstock 40 Years On: Back to Yasgur's Farm (**1/2), which means to capture the countercultural gestalt with highlights from (almost) all of the performers, including Jimi Hendrix, The Who, Sly & the Family Stone, Crosby, Stills, Nash & Young, Creedence Clearwater Revival, Santana, and Ravi Shankar.

At eight hours in length, that's a lot of music, though only a fifth of what compiler Andy Zaks hoped to include before "non-Aquarian logistical realities" got in the way, as he writes in the liner notes.

One of the things that a close look back at Woodstock reminds you is how many of the iconic acts of the '60s were not there. No Beatles, Rolling Stones, Bob Dylan, Led Zeppelin, or Beach Boys.

Otis Redding sang for "the love crowd" at Monterey Pop in 1967, but there was no similar outreach beyond the countercultural comfort zone at the 1969 Upstate New York love-in. Other than San Francisco-based Sly's psychedelic soul-rock polyglot, there wasn't a single significant African American soul, R&B, or blues performer at Yasgur's Farm. What's up with that, Woodstock?

What you do get on Yasgur's Farm are some worthwhiles, for sure. A full-of-feeling performance from Tim Hardin. A giggling Arlo Guthrie. A sweetly psychedelic Incredible String Band.

Joan Baez, thankfully, mixes overly pretty folk with surprising country-rock. And Canned Heat nearly makes the 28-minute blues "Woodstock Boogie" worthy of its length - as opposed to, say, the Butterfield Blues Band's nine-minute "Love March."

Among the letdowns: Neil Young, whose only included appearance is with CSN on "Wooden Ships" and his own "Sea of Madness." A bigger bummer: The Band, the great Canadian American realists, did not allow their music to be included.

Richie Havens' manically strummed incantations remain as off-putting as when I first heard them in the Woodstock movie, in an altered state on the Ocean City Boardwalk lo those many years ago. And then there are acts that have come up short in the test of time. Are Sweetwater, Bert Sommer, Melanie, and Quill worthy of revisiting? Only to historically obsessed nostalgists.

What's good about Back to Yasgur's Farm? For starters, the closer: Hendrix's mind-blowing "Star-Spangled Banner" flowing into the wondrous riff of "Purple Haze." And Sly & the Family Stone's deliriously joyful medley of "Dance to the Music/Music Lover/I Want to Take You Higher," which gives way to a Tommy interlude by The Who, which sounds explosive, despite Roger Daltrey's assessment of it as "the worst performance we ever did."

The real value of the Rhino box isn't so much in the music: It's as a document that captures Abbie Hoffman ranting about imprisoned White Panther John Sinclair (and getting hit over the head by Pete Townshend). And Wavy Gravy announcing that "what we have in mind is breakfast in bed for 400,000)!" And sonorous-voiced emcee Chip Monck notifying "those who have partaken of the green acid" to go to the hospital tent before the Dead embark on a typically diffuse 19-minute "Dark Star."

For non-completists, there are options. Rhino has also reissued the Music from the Original Soundtrack and More: Woodstock double disc, plus a two-CD companion, Woodstock Two.

Sony/Legacy has put out a series of The Woodstock Experience CDs (***) that collect complete performances of individual artists - the Airplane, Joplin, Santana, Johnny Winter and Sly & the Family Stone - packaged with the studio album the act released in 1969. So the Airplane live set comes with Volunteers, and Sly's is bundled with the brilliant Stand! It's a good deal if you don't own the studio albums, but you probably do. They're available together, or individually.

Contact music critic Dan DeLuca at 215-854-5628 or ddeluca@phillynews.com.