Sorry for the late reply - we were exhibiting at ASE in NY.
We did used to get DDOS'd a couple years ago. Hard. (It was well-funded by a competitor). Then we found a DDOS solution that could keep us up and it made all of that go away.
So since then we've had very little downtime. A few times here and there? Yes. A few months ago (March/April?) one of our servers kept crashing, and with load-balancing that would take the site offline for between a half hour to two hours for some people (depends on DNS). The hosting company couldn't find the issue, until recently when both drives failed. No problem - we have multiple servers, but things like that do happen.
Also, I found out just two days ago that our host did not update our DNS for the file gateway domain when they updated the main Adscend DNS to remove that bad server from the load balance. So that was causing the file gateway to be unreachable for some people and work just fine for others. I'm not sure why nobody reported it to us.
We do currently have scheduled maintenance that takes place a couple times per day and may make the site unreachable for a few minutes while it's going on. It happens at I believe around 3AM CST, 5AM, and 9AM. Something of that sort. I'll be working to speed up the process soon so that it doesn't cause hangups.
All in all though, we have third-party monitoring from 3 locations and they notify us of downtime (on the Adscend domain - which is why I didn't know about the lnkgt issue). They also compile a report for us each month which tells us how much downtime there was during that month. We always (for the past year or two) maintain 99.99% or greater uptime. Which is about the most any hosting company will guarantee. Everyone has their issues from time to time -- DirectTrack and others have as well -- but we understandably get more scrutiny of ours because of the issues from mid-to-late-2010.