I will say this, I love LawnChair - I have used it a couple of times and it makes getting data in and out of your application very easy, it is based on the premis that you need to store objects against a key, and then get it out again, it allows you to use a richer querying syntax, but in many cases (as with our IO Reader app) you don't need anything more complex.
As to why I didn't use LawnChair (or any other wrapper) in IO Reader? I actually added the data caching layer later on in the project cycle, localStorage was supported well enough and I didn't want to introduce a new dependency in the code that late on.
As a developer that likes to get things done, I very rarely write code that will manipulate a given data source directly, instead I either use a project like LawnChair, or I use a simple wrapper that I have written that allows for flexible querying semantics on JSON based objects.
I am keen to hear your thoughts. What are your favorite DataStore wrappers for Web apps? Do you write to the bare metal?