What do you like best?
Installation is very easy with Splunk. They really take the guess work out of it. Adding additional modules is also very easy. Pointing data to it is also easy. But that ease comes with a cost.
What do you dislike?
It's simply too expensive. Their price (at the time of this writing) is based on data consumption. So the more you use it, the more it costs. At the end of the day, that cost benefit just wasn't there.
Recommendations to others considering the product
If you have a very, very small shop, then the free 500MB a day license will work for you. We have about 13,000 users, and from our DC's aloe consume about 15 GB of log data a day. I switched to Graylog, built my own dashboard, and it works great. While I'm a fan of Splunk, I'm not a fan of the price. If cost doesn't mater, then you should really check it out.
What business problems are you solving with the product? What benefits have you realized?
Collecting and analyzing data with Splunk is very easy. But there are other products out there - Graylog, the ELK stack - which are free and do the same thing - just with more work. But once you build your dashboard, you are good to go - throw as much data at it as you want.