Alright, since the other reviews mostly summarize what we already know, I'm going to write down my thoughts on what I think works and why it did, and what I think doesn't work and why it didn't. I'll try my best to be as vague as possible to avoid spoilers, but I'll post warnings just in case. Characters: 6 out of 10 Levi is the Levi we know from the main manga and anime for the most part, plus a little more care and emotion. I can say mostly the same for Erwin, plus a little more coldness. These two are the ones who keptthis score up as high as it is. I think Levi's and Erwin's differences are appropriate because, in Levi's case, someone who is kinder will often be made colder after being exposed to what Levi endured, and that's what we got. Erwin is younger and slightly less mature than his main storyline counterpart, so he was realistic as well—also bear in mind that he did care more, among other things, in the Kuinaki Sentaku manga.
Isabel and Farlan are mediocre characters, to put it bluntly, and are what dropped this score from an 8. Isabel is a pretty cool character who isn't the typical "girly-girl" we often see in anime and manga, but in a story like Shingeki no Kyojin, "not a girly-girl" doesn't count for much. Such a character would be more unique and do more for the story in another genre, like romance or general slice-of-life to name a couple. She's an entertaining breath of fresh air, I'll give her that, but she's not special enough to stand out beyond giving audiences something different to see and hear. Farlan was much better in the manga than in this OVA, in my opinion. He was clever and shrewd, akin to Armin and Erwin, and that intelligence gets largely skipped over here. Moreover, Armin and Erwin already exist, and so he suffers the same fate as Isabel by failing to be special enough to stand out and hold his own opposite superior characters—namely Erwin since Armin isn't here, of course. I did care a bit about these two, though. They weren't terrible by any means—just average to a little cool.
Animation: 9 out of 10
Wit Studio delivers its best, and audiences will not be disappointed in the slightest. Seriously, these visuals were lovely to watch.
Sound: 9 out of 10
It's the Shingeki no Kyojin soundtrack, and it's great. I did think Levi's Japanese VA Kamiya overacted just a bit at the end, but it's not really an issue.
Story: 5 out of 10
This OVA provided backstory and nothing more. (Backstory can be used to drastically change perspectives on present situations in a story, but nothing of the sort was done here.) I preferred the OVA featuring Ilse Langnar because it introduced new elements that changed the way we looked at Titans and at SnK world building and its culture (later confirmed in part in the main manga). Don't get me wrong, it's a fun watch despite my griping, and it's great to learn new insights about the characters we love, but this is what one could call non-sexual fanservice. Why? Because it's really just for the fans' satisfaction only rather than the story's development—the SnK storyline does not absolutely need their backgrounds to progress—and frankly, the manga did it better. Catering to fans does not necessarily make the story worse per say, but it doesn't really add much value, so my score won't be that great as a result.
Here is where the potential SPOILER warning comes in, and I'll try to be vague: a certain scene at the end (in which Erwin influences Levi's final decision to stay in the Survey Corps) was not done justice, in my opinion. It was glossed over quite a bit. "Don't regret your decision" was mentioned, but "It's the Titans!" from the manga was omitted entirely, making Erwin seem largely uncaring and partially dismissive of Levi's situation rather than aiding Levi's resolve and getting him to see the bigger picture. Compare that to an earlier scene that features Erwin stepping out of a carriage and giving a few dramatic lines about his resolution to do what it takes. Putting the emphasis in the wrong location, maybe? That zeal would have been much better served in the final scene with Levi, Erwin. Instead you just looked dispassionate in the end.
Moreover, this OVA was pretty busy with an unnecessarily overcomplicated plot. I don't want a dumbed-down plot, just not one that tries to take on too much and show off when it isn't needed. The second episode in particular had too much ground to cover. I think that would have been aided by placing the split-off point between episodes in a different location—Levi resolving to do the "job" handed to him and his friends rather than at the point shown. I could get more specific, but that would result in spoilers. Perhaps three episodes would have been better, if it could be fit within a reasonable budget.
Enjoyment: 5 out of 10
As a manga adaptation, I say it was a slight disappointment. I couldn't help thinking that it certain characters and scenes were not being done justice, and that didn't help.
From an anime only standpoint, I say it was a fun watch, especially for more hardcore fans, but not much else. The only wow-factor for me lay with the visuals and sound; nothing else really impressed me.
Conclusion: It's a good distraction if you have some extra time on your hands, but personally if I could go back, I would have just read the Kuinaki Sentaku manga while listening to the soundtrack.