Anton Volkov’s review published on Letterboxd:
Okay....I'll try keep this as story spoiler free when it comes to the new content as I can. Possible spoilers for those yet to even see the film - and I 1000% encourage the extended version to be your first viewing if so.
The TL;DR version - best way to describe this is that it's not a completely different movie. It's a more complete, and definitely improved film. From the word go - so much more room to let the story breathe, and even by the extension everything that the story goes for is done justice better here.
The improvements: the thing about this is that it isn't just the theatrical cut with a bunch of extra scenes added on. Aside from the whole 3rd act (from end of BvS fight to epilogue - which, while being the weaker part of the film in this cut, oddly in the theatrical cut was the best paced part of the film), everything including existing theatrical cut scenes is expanded, changed up, in some cases early on even re-ordered. Most importantly, everything gets a whole lot more room to breathe, the pacing is much more even (which helps even moments like the infamous email scene which is in the same place as in the TC), and the editing and intercutting is miles better, scenes intercut and transition much more naturally and smoothly. You won't even care that there aren't that many more establishing shots even (a notable one was a nice one of Gotham early on). I was at first a bit unsure on how the wheels are spinning quite a bit but overall the very graceful build up to the conflicts is very much appreciated.
It's the same story - but it feels really quite differently executed, and for the better. There are expansions to story elements that really help make sense of a lot of the existing beats. Don't really wanna say more because spoilers, but yes, the additions help immensely.
Clark/Supes and Lois benefit the most from the new content. The balance between Bats and Supes is much more even as we get more to do, and both their storylines are really quite improved upon. There is more Supes as well, with moments that should quieten down the hate, and Bats gets a few more characterisation shades (including a, to me, very notable improvement as well, retconning a point that many at first considered to be a big problem). The new characters introduced, including Jena Malone's, while not really that big on screentime and as such easily expendable in the theatrical, are great to see and they help out overall with affecting expanded subplots. Lex is improved upon also, adding a touch of Frank Underwood as well (one of two Fincher homages, may be uninitentional) to his first-act arc with Senator Finch, and the other additions are very interesting too, but may not be for everyone due to the divisive reaction. I liked Eisenberg's take on Lex so to see more of it here was very welcome.
That third act is, for better or worse, weaker than the rest but still enjoyable to watch - probably a case of everything coming before that and the epilogue after being so good. One thing that peeves me, like in the theatrical cut, is how they tried to introduce a third act element in the build up to the big fight, but I guess all of this is something I can live with.
If you liked the theatrical cut on the first go, you'll love this. If you were on the fence/mixed on the theatrical cut primarily because of the execution issues (except runtime - because there are people for whom a 3 hour superhero blockbuster is too much), you'll likely come out enjoying this. If you didn't like the film the first time for more fundamental reasons like Snyder's vision for these characters and the places he's taking them and the overall story? You probably still come out disliking it.
EDIT: one more thing - there was something someone said about the theatrical cut being far too preoccupied with setting up a universe than it own story. Thankfully this cut does the film's own story justice for good (pun may or may not be intended).