Consider yourself warned that this movie operates on the assumption that viewers are familiar with the TV series. If you aren't already aware of the relationships between the characters (as I'm not) you're outta luck, because very little of it is explained here. And from what I've heard, things are a bit murky even then.
What makes this movie great is that it really doesn't matter. Utena is a visual smorgasborg--the backgrounds alone warrant it a high rating, impossibly lush and frequently reminiscent of art nouveau. The characters are equally wonderfully drawn. Visually, it's brilliant; the floating castle that seems to constantly rearrange itself is an astonishing feat of fantastical architecture. All in all I'd have to say this is the most gorgeous animation I've ever seen.
As for the plot...I've seen this three times now and still don't entirely get it. I have some vague theories about what's going on and why, but without the series' background I can't verify anything. The movie operates on a very surreal, symbolic level--and it's full of the eccentricities of anime, as well (not that that's a bad thing, just different--a giant car wash machine that rises out of a field of roses figures prominently in a later scene). If you go into this expecting cut-and-dry western cinema (doubtful, as by and large I expect only anime junkies would even hear of it), you're up the creek without a paddle. Even granted the contextual uncertainty, though, there is clearly a resolution. What it means is certainly debatable, but the point is that there IS a narrative here discernible in the end, if that actually matters. In this case, I don't think it does. The movie exists in its own world and can't be expected to always adhere to our rules.
Utena is awe-inspiring in its visual beauty and imagination. As a would-be fantasy writer myself I found my mind spinning off on dozens of tangents after watching it. And for one work of art to inspire another to create is perhaps the highest recommendation there is.