To this franchise overall, i suppose i have been a bit harsh on it. I went in, really expecting something near the level of the Big Key 3 of Clannad Kanon and Air (or ef and the like) but was really disappointed. Having heard all about Da Capo since forever, around forums and sites, i had regarded DC as the grandfather of VN-turned-anime-romance-drama, but i was shocked to see how little drama there was. Even now, after having seen all 4 seasons (about 70+ episodes) i still don't think there was much dramatic storyline, and i dont really see how it developed such a huge fanbase. That isn't to say, however, that it didn't have it's appeal, and im guessing that the VN delivered the story much better, which is *hopefully* where all the fans came from.
Now, my impressions on each season:
DC one certainly had very good final 5 episodes, but they wasted too much time getting started.
DCSS made much better "normal" episodes that werent intense on drama, but not lame humour, so that was good. However, it never really got to a climax for me, so that was disappointing.
DCII was also quite good on delivering well-balanced episodes of humour, drama, and a bit of romance, but my qualm with this season is the over-emphasis of the "robots co-existing with humans"-plotline of Minatsu. Frankly, i found that annoying, especially how everyone changed their minds very easily and suddenly became supportive of her. Again, like DCSS, there wasn't much climax, although the relationship between Yoshiyuki and Koko that developed from the beginning to the end of the season was very good to watch.
DCIISS was certainly the best season. With DCII having paved the way of setting the characters (and some meaningless but enjoyable filler-esque episodes, such as beach and school trip eps) this season really delivered on the drama and storyline. Seeing the characters from the first 2 DC (Sakura and Junichi) play important roles in the story, really helped connect the two "sets" of seasons together, and it ties up the whole franchise really well. The ending was alright, but the build-up to the ending was quite fabulous and emotional indeed.
I just realised i've still got DC if, and DCI&II PSP OVA to watch, so i guess im not really done with the franchise yet. And apparently next year there's going to be a spinoff about Sakura as a mahoushoujo, lol, not sure how that will go. But really, i think DCIISS tied up the plot very well, and i don't really want a further season.