"The book is better than the movie/series/whatever!" Yup, we've all heard it, and from my experience 99 times out of 100 rehashes, remakes and adaptations pale in comparison to their source material. Unfortunately the Utena manga fits into that rare, mythical 1% zone. In this case, the source material is indeed a mere shadow of its adaptation.
Not to say the manga is bad. It basically covers the first arc of the show and then jumps to the very end of the last arc, so you're getting what is generally considered to be the best parts of the series (I'm a bit of a black rose hater myself). But where's Nanami? Where are the shadow girls? Why has the blatant homoeroticism been dialed back from a strong 8 to a light 3? Where are Shichiro Kobayashi's beautiful backgrounds? Where's the WEIRD?!
The problem with the manga is all the little extra things that make the anime so special and different just aren't there. You're left with a lot of the familiar plot points (with some twists here and there, including a pretty godawful and thankfully jettisoned opening involving Utena's aunt and some nerdlinger who disappears when the story gets going), but it's like reading an abridged version of a classic. You're left asking yourself what the point of reading the manga is when you can just watch a far superior version of the same story. Then of course there's the problem with the medium itself, namely no color and no sound (two of the anime's strongest points).
It's still Utena though, and there are definite flashes of the magic the series seems to have in spades. Utena is still a badass; Anthy is still a creepy weirdo; there's still speeches about chicks cracking shells; there's still revolutions to be had. I guess it just feels kind of inessential, and...tame, which is a shame when you're used to the surreal heights of the anime.
Recommended with some caveats and an insistence that you watch the series before judging the Utena universe as a whole.