Good music. Great Latin rhythms, groovy hand drumming and batucada, blistering electric guitar, etc. Listen to many of their numbers: loads of cha-cha, samba, rumba and salsa.
I used to have some of the early albums, and various friends over the years have been fans. I don't have to mention The Zombies cover, of course. I like it all; it's easy-listening (not in the Burt Bacharach sense of the term!) to my ears. Latin-rock fusion, or whatever you want to call it, but not Prog Rock (Carlos Santana did not think so, either).
In September this year BMG Music Spain released the 3-CD "Trilogía" (82876701682) which contains the re-mastered CDs (plus bonus tracks) of the LPs Santana (1969), Abraxas (1970) and Santana III (1971). I discovered it in Madrid and picked it up for 24 Euros. It's a reasonable way of obtaining their first three albums. The Trilogía pack comes with an interesting booklet (in Spanish).