Lance has proof of it doubling or tripling the leaf size after treatements. It makes the soil more acidic too.
Of course, it should be noted that none of the evidence anyone provides to support the "coffee is magic!" principle is based on a control study. Unless you have at least two plants (one gets coffee, the other gets just water) of the same variety and measure their responses side-by-side, then this data isn't very meaningful. I think that in many instances, people who have worked hard to meet ALL of the needs of their plants are inclined to point to the coffee as the lone factor that precipitated the leap in growth, when in fact the truth is likely that it is multiple factors that bring about the improvements in growth. I'm not saying coffee treatments might not be a contributor, but I genuinely believe it isn't the only factor responsible for the improvements people are showing.