Many consider this issue to be the start of the first great Fantastic Four story arc. And in a way it is, but it's also a bit of a bipolar issue, with some great moments in both writing and art, but also some of the sillier moments too.
Starting off with the bad, the heroes act distinctly non-heroic at times. The Thing dangles a newspaper vendor above traffic by a newspaper before dropping him on a mattress truck for no other reason than he wanted to read the paper.
Later the Invisible Girl is captured without a fight and pretty much plays the victim/bait for the entire issue. The one time she redeems herself is by using her force field while unconscious(!).
Mr. Fantastic uses his powers in some real questionable ways - defying gravity to stretch at an angle out the window to his limit, catching the Thing in a pouch with his arms grabbing a building but his legs left unsecured and using centrifugal force to lower the two of them to ground, and probably the worst one: making himself into a spring for the Thing to spring off of. This stuff is worthy of Plastic Man, or maybe even Zan in the case of the spring.
On the other hand, Stan Lee shows real growth as a writer delivering, as promised, a defeat of the Fantastic Four at the hands of their enemies, and the first true cliffhanger ending in the series.
Jack Kirby is also improving, with some really great artwork here, including a full page illustration of the Wizard tossing the other members of the Frightful Four in an awesome display of his power.
There's also some other minor bonuses like Paste Pot Pete changing his name to The Trapster (which is sort of an improvement). And then there's this little gem of a panel featuring the Thing and Invisible Girl which is the 'Quiet or Papa Spank!' moment of this series.