I've tried all of the extenders out there, and the ZEN product is <by far> the most comfortable device out there. The other ones on the market tend to leave big plastic bumps in your pants if you wear them all day.
There is a bit of a learning curve involved in wearing the ZEN extender all day, but it's certainly doable.
I can wear mine for 8-10 hours a day, and the resulting bulge is natural looking. I don't use much tension, since the ZEN extender will get pulled off as I walk around. Low tension keeps everything extended all day. I usually increase tension in the evening when I'm not at work.