A great simulator, but less of a game. This will keep the budding aerospace engineer busy for hours as the software comes with tools to create your own aircraft, scenery, buildings etc. Because of the way it works the software actually simulated the aircraft, most flight 'games' just use preset files to describe flight behaviour, but X-plane actually uses the 3D model of the aircraft, so a new shape means a new flight characteristic.
However the low budget development means that some of the bells and whistles of more polished rivals (FSX) are absent. Mostly this doesn't matter and the gap between FSX and X-Plane is now very narrow in terms of polish and features. Overall I prefer X-Plane to FSX, it is in the end all about flight which is far better modelled in X-plane, and now with version 9 the view out the cockpit rivals FSX.
My only gripe is the lack of wide-screen support in the cockpits supplied with X-Plane, this will no doubt come as wide-screens are increasingly common and add realism (better peripheral vision).
Edit September 09
OK so first off I don't often get around to using X-Plane (X-P) these days. That is more to do with having two kids than any flaw with the software. What I didn't say in my previous review was that if given a choice between FSX and X-P I will always go for X-P. Add to this the fact that there are an enormous number of freeware files now available for X-P. Because X-P comes with all the software you need to begin creating planes and scenery there is a large community of enthusiasts pumping out various freeware add-ons, some of which is full professional quality. However the one downside of X-P relative to FSX is the degree to which system engineering is typically simulated. In FSX it is possible to buy add-ons like the PDGM 747 which simulated nearly every button on the flight deck. X-P is simply not that sort of software, it has a handful of active controls in most cockpits and that is all. X-P does simulate radio stacks, RADAR, autopilot, and the basic controls, but generally no more than that. Whereas FSX allows full FMU and FADEC simulation as well as complex failure mode control, basically it can allow the entire flight deck of even the most complex, highly automated, modern airplane to be fully simulated. So it comes down to what you are looking for in a flight sim, accurate flight models that are calculated directly from the three dimensional shape of the model or faithful virtual cockpits which simulate all the aircraft systems and functions. If you want to simulate flying an airliner (IFR type) as accurately as possible with all the complexity and sophistication of the real thing (but relatively tame handling) then go with FSX. If you want accurate flight modelling, airframe simulation, and seat of the pants (VFR type) flying in small airplanes then X-P may be more suited, especially if you want to have ago at designing your own planes and then testing them out.
For the majority of people FSX is probably the better choice, but if you have a deeper interest in aerospace and aircraft design, you may find X-P ultimately more rewarding. With prices for both falling I would suggest buying both - thats what I did and I love them both.