I feel Shen is more for fun than effective. Don't get me wrong, he is still a tank and therefore still effective, but not so much as the two you mentioned.
I feel shen can hold points on his own better than most champs and that teleport makes for mobility as well as a lifesaver. He is also great for ganking bot, in fact one of my favorite things to do with him was to go bot, set my ally up for the double cap and tele back up to make sure we didn't lose the point.
I actually was playing him a lot recently though recently but then I bought nautilus and switched for the past while. He's is not as good in 1v1s and may not have an instant life save, but nautilus does is (in my opinion) much more present in team fights, though not quite painful enough to be as noticeable as rammus or alistar until you start completing your build.
I guess what I'm trying to say is Shen is good, but another fun and underestimated tank if you don't have him is nautilus, and he's also worth a look.