What do you like best?
I like that Spark is an easy way to put a service up and running very quickly, plus it's easy to read and to maintain. The greatest thing is that is on Java8 which makes it more easy to read. It is so easy to learn cause it's simply that, SIMPLE. i think is bringing the best of the ruby on rails framework on Java which makes the whole process really smooth making it like a dream creating a web service so easily.
What do you dislike?
The routes part could do some work though, i know that it's a new library but it has an amazing potencial.
The routs need some work cause according to https://github.com/perwendel/spark/issues/369 the route handling is very poor and it does not matches the right one but the order as you declare the routes.
That's another issue, the issues, they are a lot and some seem to be unresolved or unanswered. seem like an eternity to get a reply of the administrator. it could use a bigger team to develop this tool
Recommendations to others considering the product
Do it, right now if your planning to create a easy to use framework
What business problems are you solving with the product? What benefits have you realized?
I use it on my personal projects to make an easy to use web server, during my internship at my current job i did have the assignment to make some pull requests to this library, i was shocked of how an amazing and huge tool is so easy to understand.
Also im developing an application that retrieves great quantities of information and return them as a recommender.