For me the sign of a good anime is one where when it ends I feel a sense of loss knowing that the story of the characters you've come to care about has ended. With Blood+ I feel this way and because of that I know it was something I enjoyed. Strangely when it all began 50 episodes ago I was very much a skeptic and initially I expected very little:
Positives: Once things get going Blood+ is a solid story about finding your place in the world and coming to understand the value of things that are most important. There are a dozen other things you could add, but for me that's kind of where it went. Over the course of the show several of the characters grow and become much more than they were at the beginning and this produced in me a deeper appreciation for the anime as a whole.
The animation is generally very good and to my relief while there is a bunch of action in this story it's definitely not an every episode all consuming affair. In fact, some of the best fights are short moments where there is little action at all.
There are also a lot of side stories going on in Blood+ and surprisingly I enjoyed them all. Another positive with Blood+ is that when the final scene came down I felt a sense of loss, but also that we'd reached an acceptable and believable conclusion. I was imagining all sorts of ways that the story could have been mangled and ended disappointingly but to my relief the ending was quite good and made the 50 episode trek worth the time.
Negatives: So the first thing I want to say is that for me the show started off REALLY slow. I almost gave up on it a few times during the first 5-10 episodes because it wasn't something I normally get into. I'm not a huge fan of vampires or the other concepts going on in Blood+ but for some reason I wanted to give I.G. a fair shake and so I stuck with it. As I noted above I'm glad I did, but be forewarned that if you're like me then you might find the beginning a little slow. Once it gets going though, you'll find it much easier to enjoy.
In the end this was a rewarding anime and I'd recommend it to a number of people, some more than others. It's not a story about a perfect world where everything works out, and there are a number of things left unfinished, but overall it's a worthy four star performance.
I'd really consider watching it a second time with the right people, and for me that's a big deal.