What do you like best?
SLI is one of those companies that are at the forefront of their field. Instead of simply providing a service, they submerse themselves in it. They're happy to make suggestions as to where to improve based off what's working in the industry. They constantly push the envelope and never say "no" to a feature request or enhancement. As a client, you get the feeling they actually want you to succeed, as opposed to some vendors who are content to simply collect your money.
What do you dislike?
It seems like the quality of the reps has slipped slightly over the pas few years. It may just be our luck, but our original rep was one of the best I've ever worked with.
Recommendations to others considering the product
Changing a site search can be a huge undertaking. I'd recommend considering SLI, regardless of the type of content you're looking to index and search.
What business problems are you solving with the product? What benefits have you realized?
Via SLI, we're providing relevant results to our searching users. We've been through several iterations of various site search solutions, and SLI has performed the best so far.