What do you like best?
We started using UptimeRobot initially because it had a great, powerful free tier, which let us spend a lot of time evaluating the service (in fact, at the time, it was *only* free). We've found it to be reliable, powerful, and (barring a strange period of time when their webhooks were HTTP GET-driven), quite interoperable with other services (through the aforementioned webhooks).
What do you dislike?
UptimeRobot is not the most beautiful looking service on the market (but the price is right and the system itself is powerful and reliable). We were also confused, a while back, when they launched webhooks but they were HTTP GET (instead of POST), but they've since added an HTTP POST option.
Recommendations to others considering the product
UptimeRobot may not be fancy, but it does exactly what we need, and for a great price. It may not have all the custom integrations with 3rd-party services that other monitoring solutions have (so this could be a deal-breaker for some users), but it's perfect for us.
What business problems are you solving with the product? What benefits have you realized?
We use UptimeRobot to keep an eye on the reachability of our various services – and on our 3rd party vendors. (A number of vendors seem to purposefully under-report their own downtime, which is understandable but not exactly acceptable when you're using their reporting tools to keep an eye on your own services that rely upon them.) UptimeRobot lets us keep a close eye on everything, and be alerted the moment something is awry.