Fallout 3 is a great post-nuclear sandbox experience.
Fallout 3 continues the great non-linear gameplay of Fallout and Fallout 2, while adding much-needed innovation to the series. The first or third person viewpoint is a much more immersive experience than the isometric point-and-click interface of the earlier Fallout games. Unlike other sandbox RPGs, Fallout has breadth and depth in the same game -- you feel like your character is really changing the world that he/she inhabits, and there are few quests that aren't worth trying. Contrast this with Bethesda's Oblivion, where a lot of quests were clearly just filler for a fun, but ultimately empty gaming experience.
VATS is a fun way to add precision targeting to the game without turning it into an FPS. It makes combat more strategic - you can spend Action Points (which regenerate slowly during combat) to freeze the game and target specific body parts to cripple the enemy's movement or attacks (or just to get in that glorious headshot).
Finally, Fallout 3 distinguishes itself with attention to little details. Some examples: Galactic News Radio, the main radio station, repeats news of your exploits to the whole Capital Wasteland when you finish an important quest. Characters talk with each other on topical subjects, and comment on your actions or warn you if they think you are going to steal the Mini-Nuke on their store shelf. Most importantly, settlements and ruins exist within the context of the game's setting, that is, I never felt like a ruined town was just there because the game designers needed to fill in space on the map.
The control system was sometimes difficult to use. Even after scanning through the manual, I only found out how to turn my flashlight on by mistake. VATS is a great system, but it can sometimes be hard to get the game to select the body part you want. I would often move the left stick in every direction and manage to select every body part BUT the one I wanted to target.
The graphics are well done, but there is a distinct lack of variety in the settings. There are only about 4 different types of places in Fallout 3: wasteland, ruined sewers/subways, ruined buildings, and Vaults/high-tech buildings. The color palette of the settings (with few exceptions) is brown, light brown, and black. Another graphical failing is the poor animation of your character in third-person view, where you look like you are skating or floating above the ground, not walking upon it.
The character advancement system is fun, but I was level 20 before I finished even half of the main quest.
Finally, the main plot of Fallout 3 is not as interesting as some of the preceding games. The side-quests (with one notable exception) are actually more fun than the main quests.
The Bottom Line
Fallout 3 is a post-nuclear sandbox RPG set in the ruins of Washington, DC and the surrounding area. Using a combination of real-time and turn-based combat (whichever you prefer) in a first-person or third-person view, you battle, explore, discover, and make difficult choices to influence the world around you.