I've read a little about this topic in the forum, and it is pretty interesting that some people "see" positive effects.
Here is a study showing that incorporating spent coffee ground has physiological effects on Lettuce. They observed increased carotenoid and chlorophyll concentrations. Biomass generally decreased with more coffee in the soil (the abstract is a bit misleading because increased biomass in 10% coffee treatment is not statistically supported if you read the full text). They interpret that the plants get stressed with addition of coffee, and caused the increased pigment concentrations (which is good for human who eat them).
Caffeine has been shown to have some phytotoxicicity. Generally, in several plants, root growth gets suppressed with caffeine. I had to read about it because I was going to use caffeine powder to control bush snails.
http://link.springer.com/article/10....738-007-0132-4 (abstract only)
Now, this doesn't mean that it doesn't work for CP. Somethings could be generalized to plants, but others may be specific to each group of plant species. Also, spent ground is different from extracted coffee.