They had some issues with their datacenter or something (I don't remember the specifics) in 2012 but all of that was fairly short and resolved ASAP. I've been with them for several years and really have no complaints. Uptime has been almost perfect (aside from those issues in 2012) and support is top notch. I probably average less than 5 minutes response time on support tickets. Many times I'm getting responses within a couple of minutes. They are totally worth the price you pay.
Some of these are false positives as once I got a notification, I checked online and the server was up. Since this is monitoring apache, there are some cases where easyapache was running and the check caught it while restarting.
I've been with WT for over 4 years now and not leaving anytime soon.