I watched Beginnings and Eternal.
They were obviously good in that they were retellings of the anime, which is already really fucking great, so it's unsurprising that these movies were fine.
Animation is massively improved, and the backgrounds are absolutely gorgeous. Lots of interesting artistic changes are made in certain scenes, such as the scene on the train with Sayaka and the misogynists having a black-and-white colour palette ala Madworld. Other stylish directorial changes are made, and the movies as a whole look absolutely stellar. Other additions include redone voice acting, all of which I thought was fine. Madoka sounds a little odd when she becomes Ultimate Madoka, but everything else was fine. There are also the transformation scenes, which are all pretty fun, though Homura's is underwhelming.
The big drawback is of course the amount of scenes that are cut out. The basic plot is all there, but lots of scenes hinting at Homura's backstory and expanding upon Madoka and her family are completely removed, among other things.
The second movie also has some weird pacing aspects, is how I want to describe it. The movie opens where the first one ended, which is fair. A scene plays post KyoSaya death that's just like 2 minutes of moving through a graveyard while some cool music plays, and we get the OP from the 1st movie with Madoka's childhood, and after Homura's backstory, we get the OP from the anime. In movie format, that's a lot of time dedicated to not much in particular, and it plays really weirdly because of it.
Most of the soundtrack is redone, and it's all mostly fine. Pretty much everything is a little worse, but it's all sufficient enough sans Mami's theme.
Beginnings and Eternal are a fun way to experience the show, but with how much is removed it's definitely not what your first viewing experience with the series should be, and I don't think it should be your first rewatch either. If you want to watch these movies, which you still probably should even if only for the animation, watch the anime twice first.