The best thing about downtimemonkey is that it helps to monitor our client's websites in real time, so we can make sure all are up at any given moment! It also has great customer support - I think they're always willing to help out with issues or suggestions made by users of their software/product.
There isn't much dislike as such; however there were some minor features which could have been improved upon (such as being able to send one-off emails from within an alert). We use this service every day for clients who host webpages through us. This means keeping everything online without issue 24 hours per day 7 days per week 365 days each year since 2010.