Still a great game, but not ideal for the Xbox
If you've never played the original Pac-Man, you are seriously missing out on one of the greatest video games ever made. It's simplicity of design, masterful balance, and addicting qualities have allowed it to endure for 30 years and counting.
The Xbox Live Arcade version is a near flawless recreation of the arcade classic (Billy Mitchell, world renowned Pac-Man master even claims it is 100% accurate to the original within 1/60th of a second - he would know, having achieved the first ever perfect score in the arcade version). This is great news for fans of the original and for those looking for an authentically vintage experience. However, the game differs in 2 key ways.
First, there is no split-screen glitch on level 256. In the original, the game would glitch when you reached the 256th maze and lock up, displaying the infamous "split-screen" level. This glitch was never considered a problem in the design of the game because the game's designer, Toru Iwatani, didn't think anyone would ever get that far. However, this glitch allowed for there to be a definite end of the game, as well as made it possible to achieve a perfect score. Without this glitch however, you can play endlessly for hours, setting high scores well past the original "perfect score" of 3,333,360. While this taints the "authentic" experience only slightly, the idea that it is possible to set lofty scores well past that original plateau is enticing.
The second key difference is the controls. The original arcade had one way to move Pac-Man - an arcade joystick. This was not like the arcade joysticks of today, with a free range of motion. Joysticks in the past were limited with a 4-way "gate", allowing for directional movement of up, down, left, or right - no diagonals. This was important because high-level Pac-Man play involved using complex patterns to run the mazes, relying on the predicable ghosts behavior and split second timing to reach the final pellet without dying. This is however very different on an analog stick. With an analog stick, there is no precise transition from, for example, up to right. It is a smooth transition - great for shooters or modern games; terrible for Pac-Man. Constantly, you'll find yourself not turning as quickly as you wanted, missing turns altogether, or going the wrong direction entirely causing great frustration often accompanied by liberal amounts of profanity. With practice these hiccups can be avoided but to set the truly lofty scores, I believe a proper joystick is required (a Sanwa JLW or Seimitsu LS-32 will do fine, as long as they're setup with their 4-way gates - and you can forget about a Sanwa JLF, standard in Madcatz TE Fight Sticks. Their 4 way gates aren't nearly precise enough).
All and all, Pac-Man is still a true classic and a great game. However, because of the control limitations, it's difficult for this to be much more than a casual distraction (although still addicting enough to make you seriously consider dropping the cash for an authentic joystick).