Shen is strong but underplayed.
Shen is lose-lane-win-game. He's a strong laner because he's extremely passive: You can last-hit at a range and easily last hit under tower due to your Q and passive. He's also basically immune to poke due to his W, with the exception of Teemo's 100% uptime poke. Later game he becomes a stronger duelist, but at that point you mainly want to splitpush and ult a teammate when you need to teamfight. However, he's also extremely vulnerable to super sustain top laners like Warwick, Mundo, Nasus, and Vlad.
Even though late game he's good at dueling, his strongest strength is split pushing and team fighting. Ulting bot lane at the perfect time means getting a bot tower and/or dragon while you only lose top tower. Landing the perfect taunt means that Ziggs can land his ult easily. He's a strong splitpusher because if he gets caught or a teamfight starts he can just ult and continue as nothing ever happened.
The reason why Shen isn't seen as often anymore is because the meta is against him along with a lot of direct and indirect nerfs. Top lane is filled with Jax, Renekton, and Shyvana, all with an interrupt ability to stop his ult. Jax and Fiora also outscale his 1v1ing power late game. Sunfire Cape and Spirit Visage, two core items, were nerfed at the start of S4. Randuins got nerfed too. At the end of S3, his E width and length were nerfed as well.
If you're on the fence on whether you want to buy him or not, flip a coin. Heads is buy, tails is no buy. If you don't like the coin flip, flip it again "for real". Keep flipping until you're satisfied with your outcome