Splunk enables me and my customers to find needles they didn't know they needed in stacks of other needles. A large portion of our solutions started in hallway conversations leading to "I wonder if Splunk could ..." and it invariably can.
In an enterprise environment of any complexity, there are hurdles with any product, but the Splunk community, as well as education and docs teams are incredibly helpful resources.
They offer trial, dev, and dev/test licenses, so I can run Splunk at home, on my laptop, as one-off testing setups, etc.
They also offer free licenses to non-profits under their Splunk4good program.
x.0.0 releases are frequently buggy, but they get patches out fairly quickly.
Splunk could really use a naming scheme makeover. (I'm looking at you, deploy*.)
We largely use Splunk for fraud prevention and security monitoring/investigation. Splunk has enabled those teams to get significantly more work done in less time with the same number of analysts. Across just two small-effort projects, we discovered fraud and inefficiencies that, once eliminated, are now saving us over $100k per month. Splunk doesn't look so expensive now, does it?