One of the most unique experiences in gaming history.
I'm not going to go into a lot of detail here. Frankly, the less you know about what you're getting into, the better.
The soundtrack is equal parts haunting, adventurous, triumphant and mournful. The plot, while initially vague and unobtrusive, is a morally challenging, thoroughly engaging, subtle emotional crescendo. The open world is gorgeous and expansive. The character animation is top notch. You have two weapons and a horse at your disposal, and that's all you'll need. The show stealers are the 16 incredibly designed colossi you will encounter and never forget.
You'll know from the very first battle that Shadow of the Colossus is destined to become one of your favorite games. Every time you conquer one of these creatures, you will experience an unparalleled sense of achievement, and, after a time, maybe elements of sadness as you attempt to understand your character's intentions. You may not think that a game which is, essentially, nothing more than a series of boss encounters could carry any emotional weight, but that's partly what makes SotC so great. Team ICO push the artistic boundaries established by decades of gaming lacking in true innovation (see ICO). As brilliant as SotC is, it's hard not to feel like each successive Team ICO production will top the one that came before. If that formula holds true, I might not be able to play another game again after getting my hands on their next release.
If you're even reading this, that means you have some level of interest in this game. That's enough. Go get it.