My rating is very much a YMMV thing, so be warned.
The Lee/Heck issues of this book suffer from the "Marvel Method" which had the artist doing a lot of the plotting; Don Heck has admitted that he was used to working from a full script and had trouble adapting (Stan Lee may not have provided much input into the plots, which wouldn't help). A lot of the plots are slapdash, padded and full of holes. Quicksilver and the Scarlet Witch suddenly lose their powers, return to their home nation and two issues later the plotline's resolved (kindly scientist who's never seen before or again figures out how to recharge them).
Nevertheless they're infused with a sense of melodrama that I enjoy about Marvel in this era as the team copes with tragedies, romances, argue about everything then apologize (Hawkeye shows real growth in this period).
Roy Thomas tightens up the plotting some, and weaves the stories in with Marvel continuity; the last story in the book has cameos by Thunderbolt Ross and the Mole Man and ties in with Captain America's then-current continuity.
Storywise, the highlights are the battle with the white supremacist Sons of the Serpent and with the Collector, a much weirder and more colorful villain than in any of his later appearances.