Let's start with the big issues. The application doesn't do landscape. Personally, I'm not completely adverse to portrait book reading -- after all, it gives you the most room to read each page on the screen -- but why doesn't Google give me the choice?
I often like flipping back and forth between my books and ongoing solitaire games (I know, I know, but that's what I do, and 4.2.1 makes it easy). Having to re-orient my iPad to the lap-unfriendly portrait orientation is just annoying. A two-page side-by-side layout seems to be a given in the e-reader world. I'm stunned it's not already in there.
Then there's the bit about, you know, actually reading the book. I experienced far too many waits as pages took forever to load. You see the image above? That's the please-wait loading symbol that appeared in my book over and over and over again. That's just insane. Why does it take so long just to turn the page? Can't the app simply cache the nearby data?
Seeing the loading message repeat so often takes me out of the book reading experience and straight into the Zone of Ultimate Irritation. Try converting from Day mode to Night mode -- it takes even longer.
Add to this slow app response the irritation involved in hovering my finger on text. I do that. It's kind of a nervous quirk. (I think Ms. Karasaris in 3rd grade taught us to do it as part of some speed-reading thing. The "enhanced understanding and concentration" didn't take but the habit persists.) When I do so, Google Books insists on zooming in on the text. Worse, that zoom is just a wee bit blurry. Admittedly, I'm not super fond of the zoom in feature, but I really hate that the zoom isn't ultra crisp.
A feature that was missing in action from my purchased book was the Scanned Pages option. When available, it lets you view the book using its original page layout design instead of automatically reflowed ePub text (called "Flowing Text" in the Google Books) settings. That's a nice idea, even if, unfortunately, I couldn't see it. That was a bit of a surprise since the book I did buy is quite recent, and you'd imagine that the application would support it for newer titles.
So in the end, TUAW is going to give the new free Google Books app a regretful thumbs down. Although the storefront may provide a reasonable alternative to Amazon, iBooks, Lulu, and other vendors, its iOS solution just isn't ready for prime time.