I really love Yotsuba! and have given a more detailed review of the series in my comments on Volume 13.

I am glad I stuck with this series because Volume 1 is probably the least satisfying of the entire series for me. The premise of how Yotsuba came to live with Mr Koiwai is strange, outlandish and seems somewhat unnecessary. Why can't Mr Koiwai just be cast has Yotsuba's biological dad? Perhaps there is a cultural reason that eluded me. Yotsuba was described as foreign, possibly alien (!?) in order to justify her naiveté about city life - however this story arc petered out pretty quickly (which is just as well).

If you can ignore the weird elements in the back story then you can simply enjoy Yotsuba as a charming story about a father-and-daughter relationship, and a joyous little girl who loves to live every day to the full.