Well, it's been over two decades since the first writeup. X-Plane is still going strong; X-Plane 12 was released recently. It has become the gold standard for home flight simulation in the Mac community, and commands a huge following in the Windows and Linux worlds as well. The 2020 release of Microsoft Flight Simulator 2020 has garnered a much larger but also more casual user base. While everyone agrees that out of the box MSFS has much prettier scenery, it's also generally agreed that X-Plane has much better flight models and systems simulation. Each can be tweaked to improve its weak areas; and thus, X-Plane can be tweaked with aftermarket plugins and datasets to improve its scenery enormously.
One of the biggest advantages of X-Plane for the more serious simulator (as opposed to someone who wants to jump in a virtual airplane and fly it around to look at stuff or do silly maneuvers or shoot at things) is that X-Plane has an API based engine which permits plugins and companion apps to affect the behavior of the simulation in realtime. As a result, you can install plugins that continuously check the internet for METARs near your virtual location and modify the in-game weather engine to reflect 'real' weather. You can hook up tablet based apps to mimic controls; you can even install plugins that will 'live' download satellite photo-based scenery ('ortho') in a 'just-in-time' manner as you fly so as to avoid having to download 150GB or so of scenery files before flying.
The flight model. X-Plane uses a methodology called 'blade simulation.' I have no opinion on whether this is a good or bad flight model. I will say that X-Plane aircraft absolutely feel more 'real' to me (compared to actual versions of them I have flown in the Real World(™)) when compared to Microsoft Flight Simulator. MSFS uses something called a lookup table model, where aircraft inputs (power setting, angle of attack, and etc.) for each aircraft are put into a 'lookup table' to determine performance. This is fast, and if you put enough effort into describing the lookup table, you can get better results - but to build the lookup table, you have to do a bunch of math or get a ton of data about the aircraft performance in advance, from real world sources. X-Plane lets you describe the aircraft shape by putting in measurements of the aircraft's shape, which is divided into sections called 'blades.' As long as you can describe the aircraft's shape accurately, the physics simulation does a very good job of then producing a more (to me) realistic simulation of the aircraft's behavior. As a result, the 'add-on aircraft market' for X-Plane is thriving. The format for adding aircraft and in fact scenery and objects is openly documented, and X-Plane makes it very easy to not only add but selectively manage your aftermarket graphics objects.
In addition, X-Plane does in fact run on all three major home computing platforms, I.e. Windows, Mac and Linux. It is native to Apple Silicon, which means that even though the Mac Mini M2 (for example) can't use an aftermarket graphics card, its embedded graphics cores are good enough to drive a very large display at 30+ frames per second even with a bunch of scenery mods installed. While PCs can drive up to 70-90FPS, it should be noted that to get this performance, it generally requires the user to buy a graphics card which costs approximately what the whole Mac Mini costs - and even then, you still have to run Windows (cough). So...you do you.
The other area X-Plane excels in is its control mappings. X-plane has a wildly extensible control input model, which means dedicated simulation enthusiasts have built insanely real to life models of very complex to operate aircraft. If you want to know what that means, get on Youtube and search for "X-Plane cold and dark" and just look at the helpful content explaining, e.g., how to start up a 747-400 airliner from cold and dark to ready for takeoff, with every single checklist item enumerated and demonstrated within X-Plane.
In case you can't tell, I like this thing. I'm currently working to study for my instrument rating, and I've built a sim rig to help me do so using Simionic G1000 units, an ultrawide 49" Samsung monitor, and Honeycomb Aeronautics yoke and throttle quadrants among other bits. X-Plane is pretty reasonable; for around $59 you get a license to expand the hell out of it. It should be noted that X-Plane is used in many cases to run FAA-certified simulations (EATD/BATDs for sure, maybe some FSS at well). While you need to buy the 'commercial' version of it to use it for that purpose, the codebase is the same - they just charge you $800 for a dongle that unlocks the program as 'supported for commercial use.'
So if you are looking to build a sim rig or a simpit, and you want to go for realism for civil aviation (X-Plane doesn't do combat simulation although you can mount guns on aircraft for the lulz) then I recommend X-Plane. If air combat is your jam, maybe go for DCS.