The 2D instruments in X-Plane 8 are very comprehensive and detailed compared to competing products, and the analog gauges deserve special mention, as they look and act very convincing when compared to their real-world counterparts. If you're interested in buying a simulator to train with that includes real-world instruments (so you can obtain an actual pilot's license), this is the product to get.
The flight model is incredibly fluid, and at its best it conveys an unparalleled feeling of bouncing around in actual air currents. This is especially true in the lighter aircraft, where landing in a crosswind is a real challenge. Occasionally, the sim reacts violently to abrupt control inputs, snapping a plane back and forth quickly in a completely unrealistic way, but overall it does an incredible job, especially considering the wide variety of aircraft types it's able to model.
Unfortunately, when it comes to overall polish and a lack of bugs, X-Plane isn't even in the same league as Flight Simulator 2004. A few times during testing, we loaded a scenario that let us perform an aircraft carrier landing only to find the carrier rapidly circling a group of buildings in the middle of a city. Several times we were plopped down on runways magically floating in the middle of the ocean, and collision detection with buildings, hot air balloons, and other objects is nonexistent. You have to put up with a lot of compromises to use this sim, and although it's worth it in the end, most people would be much happier with the more consistent Flight Simulator 2004.
2D cockpits look terrific.
If you're new to the flight sim scene, Microsoft's Flight Simulator 2004 is the superior option, as it's much easier to both use and configure than X-Plane. Moreover, X-Plane offers a staggering number of options--from dozens of graphics rendering settings to complete joystick calibration--that are likely to fly over the heads of the uninitiated. If you're familiar with your hardware and with flight sims in general, X-Plane is a fantastic sandbox that makes a terrific counterpart to the much more rigid Flight Simulator series. As a result, it's definitely worth purchasing. And even if you have a previous version, the new features in version 8 more than justify the upgrade price.