I really loved this anime, but if you plan to watch this: You need to watch both the series and the OVAs (in my opinion), cause there are some parts of the story that are missing in the series, but explained in the OVAs and vice versa. As you can see, the story is rather complicated due to the fact some things are not explained or shown in the series. For example, there's a memory flashback of an incident that should have happened some time before in the series, but was never shown, and even the flashback was too short to fully see what really happened.
The story is pretty much the same in the series and in the OVA, there's just different parts shown or explained, so if you put it together, you get the whole story. The ending is different though, I personally liked the ending in the OVAs better, for me it makes more sense (but perhaps I'm just too stupid to understand the series, I really had the feeling I missed an episode, I checked twice ^^)
The only thing I really didn't like is that the love story is quite unsolved. (Spoiler ;-) The last episode of the series (in my opinion, more like 11 1/2 than 13, as it's not at the end of the timeline) shows the memorys of the female lead, and her question to the male lead about the kind of relationship they have - and the answer is never shown... :-(