By Shizuru Hayashiya. Originally released in Japan as Vols. 1-3 separately by ASCII Media Works, serialized in the magazine Dengeki Daioh. Currently released in Japan by Shueisha, serialization ongoing in the magazine Ultra Jump. Released in North America by Seven Seas.
First off, unlike Seven Seas' Strawberry Panic omnibus, which printed 2 'magazine only' chapters at the end as extra content, you aren't gaining anything new here. In fact, you lose something, as the translation notes explaining some of the more obscure Japanese references are not included in this omnibus. So if you already have Seven Seas' first three volumes, you aren't missing anything.
If you haven't picked this series up yet, however, I do recommend it. It's just fun. Insane antics at an all-female private academy, featuring an insane School President who has girls fight in pairs with swords at random times, with financial rewards (and suggestion of more) for the winners. Into this comes our heroine, Hayate Kurogane, a simply and naive yet nimble girl, who enters the academy under a pretense - her twin sister Nagi is ill, and Hayate is there to take her place till she can recover and enter the school. Sadly, pretense and Hayate don't go together well, and this is discovered almost immediately. Luckily, the president will let anything happen as long as it interests or amuses her, so Hayate is allowed to stay at the Academy as well.
As the series goes on, we get introduced to more and more young girls with swords - in fact, it can get very difficult to keep track of them all without reference guides - and you gradually come to realize that they're all nuts in some way, shape or form. Every time you meet someone who's a supposed 'tsukkomi' or straight woman, they do something incredibly stupid to show off their boke side. You can argue Ayana comes closest, but this is a girl who keeps 'Mr. Nailbat' by her bedside and threatens loan sharks while wearing a plastic bag over her head. And this doesn't even get into the girls who are *supposed* to be funny all the time.
Seven Seas has talked up the yuri in this manga series, and it's certainly there if you want to look for it, but especially in the early volumes I think people who go into this expecting lots of Girls' Love are going to be disappointed. Even Jun, the resident pervert in the manga (who is also a kickass ninja - it's that kind of manga) is fairly sedate in these early volumes. That said, any series in an all-girls school with women teamed up as partners with swords is going to have yuri subtext, and there's some nice bits here.
I also note that Seven Seas used the cover art with Hitsugi and Shizuku for this omnibus, which makes sense. Even the most casual reader of the series tends to read through it looking for 'Hitsugi moments'. She is the aforementioned School President, and manages to be one of the funniest characters n the series, as well as easily the most badass. She is the personification of this series taken to eleven, as is her partner Shizuku, who once again appears to be the serious-minded straight woman until you see her trying to get over a cold WITH GREAT JUSTICE! They're my favorite couple in the manga.
As I noted, if you have this already you don't have to buy it again. But if you haven't checked it out, do so. Tons of strong women with swords doing funny things while having well-drawn battles. It needs more love.