Second Sight is a satisfying stealth/action title which provides good value for you if you get it at a bargain price.
For the most part, when you play as John in the present, you'll be able to use his always-increasing set of psychic abilities. These include simple telekinesis, psychic blasts and the ability to heal himself, amongst many more. When you are in the past, stealth is your best friend, as John isn't the greatest in single combat. However, he will be able to use any weapon that he picks up, and as you advance further in each aspect of the story, firearms and psychic abilities will become available to you in both timeframes.
In terms of difficulty, Second Sight isn't too hard. On Normal difficulty, I beat the game is just over 7 hours. Playing without John's psychic powers means you must hone your stealth skills, which can be difficult. Once you do attract attention, it can take a while before you are alone once more. However, that just makes it more satisfying to toss enemies around like rag dolls once you have access to your telekinesis once more. All told, the two sides to the gameplay balance each other out.
Second Sight looks and sounds great. It bears the same distinctive stylized graphics in the TimeSplitters series, and runs very smoothly with few texturing or framerate issues. The game includes a complete cast of talented voice actors, who bring the story to life while injecting so much into their characters. Empathy abounds, making it next to impossible not to care about the fates of John and his allies.
Second Sight received moderate reviews when it was first released in 2004. Five years later, it still isn't a great game, but it's definitely worth checking it out for how cheap you'll be able to find it used.