Visual novel adaptations, in my experience, tend to range from very good to very bad. 11eyes has done the impossible and achieved both at once.
Having just finished the last episode, I have to say, it looks like the animators just ran out of time when they were trying to complete the show, but still had about 30% of the game's text to cover in a mere 22 minutes.
It's not a unique storyline. Boy and friends get pulled into parallel universe where they fight various monsters for an unknown objective. It's been done before, but was much better in Final Fantasy Tactics Advance. Especially in the last few episodes, the show tends to mention things casually as though they were the most natural thing ever:
'You can travel through space-time?'
From what I can tell, playing the game in order to get the 'full experience' of the storyline might be a better option than watching the show. That being said, there's no English patch for the game, so yeah...
Animation-wise, 11eyes was consistently good. It looked clean and attractive, even on QUALITY 480p video. My one complaint about the art is that the hoard of 'Black Knights', while decent designs, had literally no detail. It was like watching flat-texture enemies. Which frankly sucked. Otherwise, it was good.
One of the things that really caught me by surprise was the sound in this show. Quite often, there are dissonant ringing sounds or booming noises that just catch your attention, because they're loud and awesome sounding, much like watching a movie in a theater when the sound is loud enough to make the room shake. Lost a point because sometimes when it was that loud, the sound actually got distorted, which could just be the quality of the videos I was watching, but meh.
I never learned any characters names other than the two main characters. They're basically the same archetypes in every popular show but with different hair and eye color. There's the kuudere sword girl, dandere mysterious magician, token annoying perverted friend and the included random girl that beats him up, happy-lovey naive main girl, yandere girl with a scary dark past, main guy who is 'stupid and reckless but has a kind heart', etc etc. Same old.
This show did catch me by surprise at some times. At those times, I said 'Hey, this is great'. Until the last episode, when everything was rushed, glossed over, and never properly concluded. There is going to be an OVA in the summer, but it seems like it will have different content, since apparently the main storyline is over. I'm hoping it takes some time to explain some of the 'casually mentioned in conversation' things such as time-space travel, Abraxas the spirit, and other things I won't mention to avoid spoilers.
Good but stale.