I can only speak from my own experience. I found the services I tried to be of little value - they were not accurate. Most likely problems between their servers and mine etc.

I actually found the same accuracy and more control using "Radar Website Monitor" and leaving my system on 24 hours...

After about 6-months...I finally decided the host that manages my server was doing their job ;)

I don't think we have a summary here of the available monitoring services like we do for web analytics.

Anyways, I'm using a free one from basicstate right now.

The faq has some pretty good info on what to look for.