I am finishing my second pass through Athenaze II.
As stated by others here, it is much harder. It's almost like the authors ended Athenaze I, and went into a slight panic when they looked at what they had covered so far, and threw everything but the kitchen sink into the next book.
I have absolutely no regrets about going ahead with book II, but I did get discouraged and worried when I first started reading passages from Thucydides and Herodotus and struggled, especially since the book one readings seemed so smooth. A number of the stories were sort of odd (such as one about two sons who pulled their mother in a cart six miles to a festival) or complicated. Frankly, there were some that I only partially understood, but that got much better on my second pass through the book.
If you decide to go ahead, be sure to get the workbook along with the regular textbook. It is self-correcting, which is essential. As previously mentioned, you will need to plan that it will take longer per chapter. Even on my second pass, it took me two weeks or more weeks per chapter. Each textbook chapter will give you new vocabulary, multiple readings to go through, several grammar sections, exercises and at least one of those historical enrichment readings. The workbook will give you exercises and two readings from Cebes' tablet per chapter. At the end of the workbook, you will be presented with the additional gift of 10 supplemental grammar sections, plus exciting exercises for each.