Mike,
I have had the best of tanks over the past 4 years with a mutitple filtration system (plenum, sand bed and skimmer). This time in the 180 (in the wall) I am going with a shallow sand bed. In the event I used a plenum again, and it did crash, I would have a heck of a time getting the gunk out without trashing the whole damn thing.
I have seen many, however, successful tanks that have been up with plenums for years (some 10)
ONe recommendation I found, that was the only one that made sense was to insert an airline tube below the plenum so that once a year or so, you could suck out the nasty water from under the plenum.
Anyhow, they worked great for me. I have one in my 92 corner and stuff grew out of control in that tank. It is still up - 2 yrs this past July.
House