It is similar but still different - the gameplay is now more free - you don't play story missions only - instead you choose which mission you want to do - you can choose to do only random missions for money or missions that progress the story.(missions can be espionage, theft, assassinations etc)
The controls are much better now then they ever were in tenchu (though the slow camera movement is still a pain in the ass).
Also there is an alchemy system - you can mix different ingredients and keep improving mixtures that you then can use on tools - different things are possible you can use effects like sleep, stun, heal, strength, explosive etc on tools like potion bottles, sushi, spheres to create usefull items - like spheres that make people sleep or explode (or both) - or sushi that explodes when eaten by a hungry guard etc
I'd say the game is like a better tenchu - if you thought Tenchu was ok you'll like Shinobido.
edit: oh I forgot to mention that you get to design your own ninja garden with traps and decorative stuff - and from time to time you have to defend your hut when its attacked by enemy ninja clans etc.