Instrumental is a very low-drama tool. It's focused on gathering metrics, making graphs and dashboards, and doing some simple alerting, and it does those things well.
We use it to collect metrics at both the OS level and the application level. When all of that information is in the same system, it's far easier for us to correlate causes and effects in ways that would require multiple open tabs in other systems like New Relic.
I hate "Contact sales for pricing" links when I'm evaluating new additions to our tech stack, so I appreciate their simple, straightforward pricing model.
Their team is also pleasantly proactive and available. At one point they contacted us because our number of metrics had grown, and they wanted to make sure it was intentional. Other times, I've been able to have a quick chat with them about a desired feature or a library question, and it's easy to tell that you're talking with the person who wrote the actual code.
I can't think of any real negatives. They don't have libraries for every language, but they hit the major items I've needed. In the one case where I had to write some code myself that called their API, their documentation made it very low fuss.
Instrumental made it easy for us to get simple metric collection implemented without running something like Graphite in our own infrastructure or turning to expensive contracts with a service like New Relic.