I've indexed a few books already under the trial program. On the whole, it's pretty straightforward and the interface is pretty easy to use.
Please be sure to read carefully the indexing guidelines and manuals -- EYB has rules and styles for listing recipes, categories and ingredients. If you don't want to have to go back and do lots of corrections to a book you've indexed, read these guides first! Have a good look at the list of recipe categories (by clicking on that link in the manual) to see how EYB categorizes them. It has many examples listed, especially of recipes that may differ by country (for example, flapjacks in the UK are not the same thing as in the US, so different categories). There are also guides for using the "Occasion" and "Ethnicity" fields. Read the guidelines for listing "Store Cupboard Ingredients" which can trip you up, and print out the handy shortcut abbreviations for entering common ingredients.
It sounds like a bit of a hassle, and it does take a bit of getting used to, but it's pretty quick to learn the EYB rules and guidelines and then you'll be indexing away like mad! It'll really be great to have lots more indexed books available!