I've been on certain anime forums and Guilty Crown had a lot of negative feedback during the first half of the series then it went downhill as the second half started. I personally liked the first half while the second half is just bearable. Nevertheless, the whole series is still entertaining. It just depends on your preferences, I guess. The ending is sad to some extent since there are losses involved, but the buildup to get to that point is kind of lacking.
You'll probably love the song that's going to be played during the last scene of the final episode. That particular song is now a favorite of mine. But overall, Guilty Crown has some of the best OSTs from My Dearest to Euterpe, etc. The second opening song is also nice so look forward to it!