I have finished the book but I can't really answer your questions. Basically I went through it because I was interested about the audio. Certainly it has about the best reconstructed Greek audio I'm aware of (or actually I think one of the four voices was very good, one the two men – but he was so good I think it made it worthwhile to go through the whole book, although the others were not so good).
But to answer your question – I don't know. I already had the target level when I started it. But basically I think it's a good book, a very practical and pedagogically sound approach. You learn pretty much Greek but the book skips the theoretical parts, so you learn intuitively. But if you've gone through 30 lessons, you probably already know what it's like and whether the approach works for you.
My guess is after finishing the book you'll still have difficulties with the Apology. But if that's the way you want to go, I think Assimil is as good as any other, or better. I can't claim to speak from experience, but I suppose the time will be well spent. Greek is difficult whatever you do. Maybe later on you want to complete with another more theoretical book, but you don't have to do everything at once. First learn some Greek, then learn the metalanguage, or something like that.
But I think it's a good approach to get to read real Greek as soon as possible. With Attic, there are not many easy texts unfortunately. I think the Apology is one of the easier ones.