I like how easy it makes finding out if your site goes down or not! It's really helpful for any business owner, especially with my work as an IT administrator in school system who has many sites throughout our district which need monitoring regularly since they are usually live 24/7 due to accessibility of schools during non-school hours (i.e., after 7 pm).
Sometimes there will be lag times when using some apps such as Google Hangouts but this doesn't seem too often anymore than normal websites do these days so no complaints here just keep up good service otherwise everything should stay fine. We use uptimemonitor at all three campuses where we host multiple applications including one virtual classroom software provider website among other things along side various local web services hosted by us. The biggest benefit right now would have been from being able to see what was coming into each application server via IP address rather then having someone go check them manually.