I'm sure I'll simply be restating what many others have said, but I still want to share my piece. I really liked Usagi Drop, from start to finish - that is, I like it when I put on my splitting goggles and tell myself that it was two totally separate stories/series.
The topic, character development, story line, dang - even the writing style - was completely different between the first half and the second half. I love both stories, they each have their charm. But they should not have been in the same series. They were just two different - the shift in story type from the first half to the second was just too jarring, leaving the reader feeling disjointed and uncomfortable. I absolutely ADORED the first half - I felt it was very unique, enchanting and heart warming, and I feel I would have loved it even more if it had ended in the first half where it was - it would have made it more captivating, allowing the reader to fill in the future however they liked. The second half, well... I agree with the others; it was inferior quality and at the end of the day, not a memorable piece. But, nonetheless I enjoyed it as a time-passer.
Sigh I will definitely suggest this manga to others in the future, but I will urge them to read each half at least a few months apart, so that they are their own separate (and beautiful) soloists rather than attempting to dance together with two left feet each and no rhythm.
I give the first half a 9/10 and the second half a 5/10