Watch it from Internet Archive here:
1000 Misspent Hours has this to say towards the end of their review:
The movie sets itself apart from the rest of the pack by playing the Sympathetic Monster card, and does it quite well, at that. It’s not the Ymir’s fault that it’s a menace, after all— it would have been perfectly content to spend its life on Earth hanging out at the sulfur pits of Mount Aetna, never bothering anybody who was smart enough to leave it the hell alone. It’s also just a really impressive monster,
Harryhausen’s effects are highly impressive. These represent the real blossoming of him as an animator from the point he was merely creating monsters to where he started to craft them into characters. There are some impressive scenes with The Ymir fighting a dog and battering a farmer – 20 Million Miles to Earth is quite a surprisingly violent film in some regards.
DVDJournal opens their review with this:
Of all the Rampaging Monster B-movie potboilers from the 1950s, 20 Million Miles to Earth, showcasing the work of stop-motion animation innovator Ray Harryhausen, remains among the most fondly remembered. And rightly so.
a favorite 50s monster film and the last Ray Harryhausen effort in B&W. Ray's animation talent and special effects compositing skills are shown at their best.