[yuriyuri]

I tried using this book for my self-study but didn't get in as far as you are now, but since your asking for opinions I will give mine.

I thought it was too expensive for what it was, it wasn't engaging at all, there were mistakes in it, it used romaji in the first chapter, for me it felt as though they were holding back on the kanji and the sentences in general didn't sound like something Japanese people would say.

On the last point about unnatural sentences, I think your bound to get them in all textbooks made for English speaking learners of Japanese, just because they end up trying to force English ways of speaking into Japanese a lot of the time for some reason, so I'm not sure how difficult it would be to get away from that.

I've heard good things about みんなの日本語 but I have never used it myself.
Then again the whole reason I bought Genki was because I heard good things about it.
The best thing I did was give the textbook away to some other guy who wanted to try learning Japanese.
I then bought the small book "All about particles" by Naoko chino, and just read Japanese blogs, news sites, mangas etc.
It was more fun, and I could look up grammar when I wanted to very easily.

Basically after readng through a few chapters of Genki it put me off textbooks all together, so I am biased towards using things like Tae Kims grammar site for example.
Of course some of his example sentences are also incorrect or unnatural but at least I didn't have to fight my way through a chapter on how to tell the time just to learn one use of the は particle lol.

Anyway.
In short, I don't think Genki is all that great of a textbook.
Unfortunatly I can't give opinions on any others.
If you feel you absolutely must have a text book then from what I read of Genki when I tried it I would have to say, try a different textbook and see what happens.