They're fun to play and not too entirely annoying unless you fail over a hundred times. For each boss battle, there is a "Todome", a QTE which usually involves killing the boss in a spectacular fashion (my personal favorite is the Thunder Hydra, where you just take dual blades and rip its mouth a new one). The QTEs (Quick-Time Events) are a direct spinoff of interactive cinematics from RE4 and 5, and they seem to have gotten it down just right, though the game is a bit lenient, allowing you to start off from a certain point in the QTE should you fail, instead of the beginning. It's mostly used to reflect projectiles, attack air opponents, and act as a defensive shield due to its long range abilities and speed. The Dual Falcon Blades do the least damage, but are the fastest. It's slow, but does the most damage, and later upgrades allow for abilities like whirlwind and charging smash. The Stonereaver sword is used to knock down weak walls (identified by your eyes or Ninja Vision), and also smashes the shields of armored opponents. The Oni-Slayer is your all-around weapon which balances speed and attack, and when later upgraded has deadly launch-into-air combos and charge attacks. In Ninja Blade, you have 3 different weapons to choose from: the Oni-Slayer Blade, the Stonereaver Sword, and the Dual Falcon Blades. For ranged opponents, you can use your weapon and hit back projectiles at the enemy, a nice change from standing, blocking, and then running while they reload. Killing blow combos are fairly straightforward, only one or two per weapon, but always look incredible when done right. The shurikens are also a nice touch in combat, having area of effects, including on some bosses, where you have to use the shurikens to disable their invulnerabilities. It's all your usual hack-and-slash fare, with each weapon having its own combos to pull off, and with varying powers and attack speed, making them fairly well-balanced. Now, this is why you came to play this game, isn't it? You're a ninja, you got all these cool moves, so you're going to put them to good use.
The game does have a tendency to lag quite a bit at some times, but it's neglectable. The visual effects of the game also play a big part in making you feel all cool while you go shinobi on some poor bloke's body. Sure, it's not to compare to next-gen standards, but they don't look too bad either.
This might not be its best selling point, but the game really does look good. So how exactly does one pull of a good ninja game in a modern day setting? The guys at ND Games sure know how to.