I'm not a big reader of comics, though I do occasionally like to dabble. The idea of this I found intriguing - an alternate history of a fictional, and yet familiar enough with a big enough back story and enough cultural cachet to justify an alternate history all of his own.
Essentially, without damaging the enjoyment of anyone that wishes to read this for himself, Superman arrive on Earth 12 hours earlier than he would have in the established mythos and becomes a more literal Man of Steel, next to Stalin (which, of course, means Man of Steel. Also: not his real name). The Superman character is, as I've come to expect, a good man. In spite of the system he's working in, he attempt at all times to do the right thing. So, whilst the outcome of much of what Superman does is less than good, his intentions are uniformly good. He really believes in socialism and wants it to work - I think Millar handles this quite well - it would've been easy to make him the "bad" Superman, but he resists this and thus keeps the character sympathetic.
The comic is then an entertaining romp through the DC universe with many other characters making an appearance throughout to side or battle with Superman. I particularly liked the Soviet Batman (and this is really a personal political quirk of mine - I like Batman! Really! So no implied criticism here) seemed a little less of a reactionary in the context of the Soviet system (he fights purely for people's liberty in this - he's not a rich vigilante - though I realise that now Batman has a rich back story now and there is more to the character than that). It includes also, naturally, Lex Luthor, an un-assassinated JFK, Hal Jordan, Wonder Woman and many other people real and from the Superman universe.
As far as I can tell, it's written in a good comic book style (so rattles on at a fair pace) showing respect for its source.
My only criticism would be that as, I believe, it was originally only 3 issues, it's probably a tad on the short side. There's a lot of stuff going on on this, all of it highly enjoyable.