There already has been some outrage expressed over the switch to new APIs; five months is not a lot of time to shift gears if you've built a significant architecture on top of the old APIs. But I have some sympathy for Google, in this case; the new "Discovery" APIs are based on a common, consistent architecture that will be easier for them to document, maintain, manage, and ... monetize, of course. (Good time for full disclosure, I suppose: I am a Google stockholder.)
So far, the only major concern I have with the new v1 Google Books API is one missing function that was available in the Data API: the ability to do a full-text search of a custom bookshelf. Accordingly, I've filed a bug in the AJAX APIs issue tracker. Here's hoping that the deprecation of the old APIs enables Google to focus on their anointed APIs on all fronts: documentation, features, and support. Bug 587 should be a good testcase.