Tl;dr - blah blah blah, I just kept waiting for something interesting to happen. Then it was over. That's what she said, I know.
Seems like there have been a lot of shows about high school musicians recently. Shigatsu wa Kimi no Uso was pretty great; it's part of the reason for this sudden explosion of a genre that was pretty limited before. Hibike Euphonium takes a little bit different approach and focuses on a high school band and their preparations for a competition.
It has great production values and looks gorgeous... my only criticism on this part is that sometimes the music is too good. The real life band that plays the music either sounds like a full-blown professional band or a professional band that is purposefully playing badly, depending on what's appropriate for the scene. To continue the comparison, Shigatsu wa showcased some really impressively diverse piano and violin playing that sounded believable (to me at least), whereas Hibike Euphonium is pushing my suspension of disbelief a bit. It's not a huge issue, but hey I get to hold you prisoner to my words here. In any case the art is gorgeous.
Unfortunately the plot doesn't keep up with the technical production quality of the show. All the characters and events are just so wishy-washy and dull. Really they're mostly acting pretty much like real life high school students, which is the problem. Like most real people, the plot always seems hesitant to commit to any particular direction and instead mostly comes across as a mush of undeveloped plotlines. So yeah, basically nothing happens.
It's not bad by any means, but there isn't really anything worth watching. Nothing happens, nothing happens, The End. Maybe if you were in high school band you might get a nostalgia trip out of it, but the music isn't interesting enough for a music lover (which I am) and the plot is pretty dull. Slice of life is fine and all, but show me a slice of the life of someone interesting. As it stands, Hibike Euphonium gives me the same sort of feeling as Farming Simulator 2015, so I don't really feel like recommending it.