I like that it monitors my site for me, so if there are any issues with speed, downtime etc., then they're notified immediately.
This also means i don't have to worry about checking up on things myself! If you want peace of mind over your sites performance, this service is perfect. There isn't anything really negative in my opinion - just some minor niggles (i'm sure these can be solved). It does what it says on the tin, which is monitor my websites' performance and notify me if something happens (if its not working properly!).