Story: The story of Shugo Chara! Doki!! Is essentially the same as the original Shugo Chara, Eggs appear for Amu to purify blah blah blah for endless fillers, but I found that this series seemed quite a deal more satisfying than its predecessor for two reasons:
Reason One: The drama. Instead of being 100% innocence and cutesy, this season of Shugo Chara seemed to have an added oomph what made it appeal more to me. It also made it seem for an older target audience, another nice improvement from my point of view.
Reason 2: The romance. Enter the Tadamu/Amuto love triangle what everyones been waiting for with bated breath, what suprisingly goes on for a large selection of episodes and has some interesting suprises. Amongst certain love confessions and certain cat-boys sleeping with Amu (Interpret that how you want to ;)), this is the main part of the show what keeps you watching.
Yes, admittedly the very start of the show can drag on a bit, but when you get to the middle- all the way through to the end it picks up considerably with some good improvements and interesting plot twists.
Animation: Identical to Season 1, the animation was very good, but could turn sloppy in a space of a second or two.
Sound: Also exactly the same as Saeson 1 the music of Doki!! isn't that memorable, but its by no means bad. The music suits the moods of all the episodes and the opening and closing songs were very catchy, tempting you to download the OST.
Characters: Quite an extraordinary amount of character development happens in Shugo Chara Doki!! And many secrets are revealed in the process of the revelations. Tadase and Ikuto in particular are developed a great deal in this season as well as the new character of Lulu. The only flaw I can mention is the one of Lulu's character development which took around 40 episodes to happen. They started telling you her story, but then they would pause and continue in about 20 episodes later which could get rather frustrating.
Overall: Definitely watch this if you liked the original Shugo Chara and want to see more romantic movement in terms of Amu, Tadase and Ikuto. The story isn't particularly back-breaking by any means, but the drama and romance makes up for it.