I definitely can speak the opposite about Uptime Robot. I've not had good luck with them. I tried it out and it seems to have issues monitoring the websites I used. I have 4 different monitoring softwares/companies monitoring our servers, and UptimeRobot would randomly tell me the site is down. I'd check it out and could get to it without any issues. A while later (many times over 15 minutes), I would get an email stating the server is back up. If the server was truly down, I can be assured that clients would have noticed a 15 minute unexplained downtime issue.