With Kirby gone and Lee's attention elsewhere, the Fantastic Four get into some pretty dire material. John Romita took over the art in Kirby's wake, handling issues 103 through 106, which is probably the nadir of this period. Starting with 107, John Buscema takes over and things marginally improve while remaining quite poor. Most of these issues feel like Buscema trying to swallow the shadow of Kirby & Lee and come out with something different on the other side. Issue 108 builds a framing story around some un-used Kirby art and is a literal example of this. Issue 110 even has the audacity to include a knock-off Kirby collage!
Storywise, this is mostly half-assed rehashes of things we've seen before with lots of the FF acting out of character for either no reason at all (issue 107) or dumb reasons (110-112). The three primary arcs involve the Negative Zone (the worst of the three); the Thing turning evil, battling the Hulk (you'll never believe how boring this battle is), dying and then immediately returning to life with no consequences; and the FF battling the Over-Mind.
That last one sees Buscema joined by Archie Goodwin on script. With Lee relegated to plot & editorial, we get something different and actually pretty good. With issue 114, Buscema stops aping Kirby and draws in the Marvel house style. In issue 115, Goodwin starts scripting and we get a pretty solid space opera origin story for Uni-Mind and the Eternals. In issue 116, things gel for the first time in ages, and we get a good story with Evil Reed Richards and Dr. Doom leading the FF. It's fresh and brings this whole collection up to the two star level.
One other highlight is the Thing wearing sandals in 105. Love that look.