The entire "Galactus trilogy" is presented in this superb (and almost perfect) episode. Of course, there are some short-cuts or edits, but the plot of the "trilogy" is still there, and is probably the best animated adaptation of the classic Galactus story. In fact, not counting the '90s adaptation of "The coming of Galactus" (which was twice as long as this one and three times as worse, in my opinion), it is the only animated adaptation of the classic Galactus story.
The animation looks a lot better than in previous episodes (especially the Watcher's holograms, the Human Torch's journey through time and space and of course the fight between Galactus and the Silver Surfer), and there are some incredible characterizations: the Silver Surfer learning once again what emotions are, and Galactus who is considered being "beyond Good or Evil", by the all-knowing Watcher. The Watcher is the only character introduced in this episode who isn't given a real characterization, but I guess that's understandable.
Galactus however is (apart from being colored strangely) portrayed perfect: he does not care if the Earth will be destroyed as long as his hunger will stop.
Also keep in mind this is a '60s animated show, so there are bound to be some goofs or animation errors. I am talking of course about the fact that the Watcher's transportation device looks just like the Ultimate Nullifier. Overall, this was an enjoyable episode, and deserves the superb score it has.