A failure in many ways, a success only in one.
Shadow of the Colossus is not that. Each boss battle has some sort of trick to it. A puzzle, if you will. Solve that puzzle, and you get an opportunity to mount the boss. From there all you have to do is climb the creature's furry parts until you get to a shiny part, which you then need to stab a couple of times. After a couple of stabs the boss' life bar will be depleted, and he will fall to the ground. That's it. That's every boss in the game.
"But Mr. Reviewer," you say. "Surely these puzzles are complex affairs that require a great deal of creative thinking?" Nope. They're all incredibly simple, and solving them is as easy as finding a single standout feature in the environment or on the colossus and exploiting it. "But Mr. Reviewer," you say. "Surely the furry parts you need to climb offer amazing platforming challenges?" Nope. All you do is hold down the R1 button to stick to the fur, move with the analogue stick and jump with the triangle button if necessary (in most battles, it's not, and when it is it requires no skill of any kind). "But Mr. Reviewer," you say. "Surely the stabbing of the shiny parts requires immaculate timing?" Nope. The colossi will occasionally try to shake you off, but all you need to do is keep holding the R1 button until they stop -- which they usually do long enough that you can wait a couple of seconds and still have time to attack -- and then start stabbing. Should you happen to fall off, you can just climb up again right away to try once more.
The only thing that saves this game from being a complete failure is the presentation. The orchestral soundtrack is powerful (though the way it fades in and out as your distance to a colossus changes can, at times, be jarring), the vast, empty landscapes are quite beautiful, and the colossi are some of the most imposing enemies ever seen in a video game. Some clump around two legs carrying enormous weapons, some walk on four as wild beasts, some swim in the depths of dark lakes and some soar through the skies, but they all have one thing in common: they are, as their names suggest, absolutely colossal monstrosities of fur and ancient stone. The ground trembles with their every step, and approaching them -- not to speak of climbing them -- is quite the audio-visual experience.
Unfortunately, even the aesthetic side of the game isn't without its issues. The game runs very poorly, and it's rare to see anything resembling thirty frames per second. It never ventures into the realm of the unplayable, but the poor frame rate can when it's at its worst be fairly distracting and take away from the experience quite a bit. Also, the use of bloom and blur (presumably to conceal the primitive environmental textures) is excessive, and hurts the presentation more than it helps it.
Hopefully, one day other, better developers will try their hands at making boss-only games. Hopefully, those games will be amazing. Sadly, all we have for the time being (that I know of, at least) is Shadow of the Colossus: a failure in many ways, and a success only in one.