A very nice, well-optimised, complex and realistic space simulator. X series has always ruled in space sim genre and Reunion is no exception. It has a really huge universe with tons of AI controlled ships, each minding their own bussiness, trading wares between space stations, which affects the prices, thus the economy is quite dynamic and feels real. If you destroy a trader ship, it will not arrive to it's destination and the station will not receive the wares it needs to construct something. So yeah it feels a bit real and your actions do have an effect. But this all was just the core feature of X games. It's soundtrack is astonishing, as in all the X games. Alexei Zakharov did a good job again. You will notice many tracks still being ported from older X games, but I love the new ones, especially Home of Light and Kingdom's End sector music.
So what's specific for Reunion? Well first of all, I enjoyed the older X2: The Threat way more. It's atmosphere was just a bit different, it's userface-control was faster, where you weren't sometimes forced to use a mouse and could do everything quickly through a well-programmed flightstick/gamepad (which I really recommend for X3:R). It's a-life was also better. Each station had a set of fighter and trader ships, with their corresponding orders. Here in Reunion, everything is replaced with universe traders. I couldn't find a station that owns at least one ship, I thought it's a bug. It only feels better because the ship names look like "Arms dealer", "High-tech transporter", "Casino", "Civilian ship" etc. But they all just have very simple roles and you can't buy anything at an arms dealer. I really don't like how unminimalistic the userface in Reunion is, it just lacks propper organisation and feels really confusing for a newbie. And the windows could be more scaled up a bit. The menu also got some graphical bugs on widescreen monitors (not breaking it's usability), but ingame there are no widescreen fragments. And if you're an X2 player, say good bye to station interiors. There are only very few of them, mainly at trading stations. Ships dock from outside, into a limited number of ship hoders. But the capship capacity is increased at least.
Don't forget to look into Egosoft forums, you can find many awesome mods in there! X-Tended Mod (XTM) is probably the biggest one, but if you are gonna play TC, then it may spoil some things for you as it ports the ships and stations from that. The cockpit mod is also really good because there are no cockpits in Reunion, as oposed to X2. Please note that it isn't compatible with XTM. Also New Horizons seems like a nice, non-spoilish alternative to XTM. And there are many good plugins like CMOD II, adding new sounds, Military HUD, I want to go fast (because ships in X3 are generally slower when compared to X2), etc.
If you're really really into gameplay and want to enjoy a little bit more realistic universe with it's own unique atmosphere (not saying the Reunion's one suck btw, it could be like comparing Gothic 2 to 3), go for X2: The Threat. If you don't care about the story and can't deal with old graphics, X3:TC (with AP after it) is the better one for you. If you haven't played the previous X games (at least X2), then Reunion isn't really an ideal start (as it doesn't even have a tutorial) and I would go for TC aswell.
And talking for the whole X saga, Reunion is probably the worst one. The only good thing it brought to the series was even more awesome graphics, which are still eye-candy to me. But it's story is long enough (longer than X2's) and every X-fan should play it