The controls on the PSP work fairly well, and as you might expect they're very similar to the console Splinter Cell games. You move with the analog stick and use the face buttons to perform various actions like crouching, jumping, and interacting with objects. The directional pad is used to manage inventory, pull out weapons, or switch vision modes. When you have a weapon out the camera pulls in to a first-person view where you aim with the analog stick and move with the four face buttons.
The biggest problem in the translation of the console control scheme to the PSP is the camera control. By default you control the camera by holding the circle button and moving the analog stick. That means that you can't adjust the camera while you're moving. Instead you have to stop what you're doing, hold the circle button, and move the stick around in an attempt to see what's going on. It's extremely frustrating, and at best you'll only ever find an almost-decent camera angle. There is an option to control the camera with the face buttons, which has the benefit of allowing you to move the camera on the fly, but it's still clunky because it requires you to switch out of camera mode to perform any actions.
Even if you do manage to get a view that isn't obstructed by a wall or the back of Sam's face, all you'll usually see is darkness. It makes sense that a stealth game would rely on light and shadow, but Essentials is so completely shrouded in black that you'll rarely know what you're looking at, if anything. You can remedy this by using the night-vision goggles, but those are available only in certain missions. So while Essentials might look good, you'll never know it because you'll either be looking at complete darkness, or getting the grainy greenish look afforded by night vision. Fortunately, the sound is almost good enough to play by that alone. Audio cues such as footsteps go far to improve your situational awareness, especially if you play with headphones.
If you get tired of struggling with the camera in single-player mode, you can always share the struggle with a friend in spy vs. spy mode. This is basically a two-player ad hoc deathmatch on one of four maps. The average multiplayer match boils down to a knife fight in the dark, which is every bit as clumsy and unskilled as it sounds. In multiplayer mode you do have access to the night- and heat-vision enhancements, but they're limited by a battery that has to recharge every 30 seconds or so. To make matters worse, the frame rate occasionally drops to unplayable levels and/or momentarily freezes altogether during the multiplayer matches.
Splinter Cell Essentials sounds like a fine idea. Take some missions from previous games, mix them up a little, add some entirely new missions, and fit it all onto the PSP. Unfortunately, due to some bad controls, oppressively dark levels, and a worthless multiplayer mode, the result is a game that is more frustrating than it is rewarding.