To put it bluntly, the OVA doesn't do the game justice (if you haven't played it, go find a copy of the non-18+ version and play it right now! Do it!). Part of this is understandable; the OVA appears to serve as an introduction to the plot only, but even in doing that, it falls short. There are too many key details left out, and the processional of events is slightly different as well. Game aside, the OVA is very weak even as a stand-alone animation. Viewers who have never played the game and never intend to will have no grasp of.... anything. To these viewers, all they'll see is some people pulled into another world, swords clashing, magic being cast, with few reasons or explanations behind what is going on beyond the typical cliche. Suffice it to say, if you're not one who appreciates what Japanese doujin/visual novel games have to offer, don't bother with this one.
For those of you that do! Let me take this opportunity to strongly recommend obtaining a copy of Eien no Aselia: Kono Daichi no Hate de and playing through it at least once. With over 50 hours of play time (not including replays for different heroine routes) and a wonderful story and plot, the game ranks high up on my list with Fate/stay night and Kanon, with actual gameplay elements beyond the typical read/make-a-choice themes that we usually see in these kinds of games. Haters of eroge, fear not, there is also a non-18+ version out there, depending on where you get it from. Best of all, an english translation is out, courtesy of Dakkodango translations, so non-Japanese readers can enjoy the story too.
Ahem.... anyway, the gist of it is don't bother with this OVA unless you've half a mind to at least try the game. If you do take an interest to my recommendation above, you might perhaps watch it as a way to get to know the characters you'll eventually meet. Don't expect any epic storytelling or anything from the OVA beyond that. If any of you all have any questions regarding the game before you potentially throw 50+ hours of your life away playing it, shoot me an e-mail or message and I'll answer your questions.