Not horrible, just brutal to start of with. Once you get the hang of the controls and some money in your bank, it can become addictive. But the story is bad.
X3:
+Pretty
-The slowest game in the history of mankind, just finishing the weak story will take hundreds of hours where 99% will be just waiting to arrive at your desitnation
-A severe lack of information on how to play the game and how to accomplish missions makes the learning curve more like a wall
-The only actual gameplay is shooting stuff and that isnt much fun
-Really bad control interface, you'll need to click deep into menues for simple basic stuff
-Its really boring
-The missions can be extremely frustrating because of insufficient ifnromation and bad contruction. Its also very easy to break them.
I hate it, I really do.
I've easily put more hours into X3:R than any other game I've ever played, only Ultima Online could possibly give it any competition. It would be helpful to know if you're talking about X3:R or X3:TC. Some of the complaints, look more like TC, as of a couple patches ago, don't know how it's doing currently, I've been too busy to load it up. Some of the criticisms are entirely justified though. The manual and story aren't really worth bothering with for the most part. Though the forums and the guides you can find there fill in for the manual reasonably well. And the interface could use some work, but if you spend enough time with it, I at least stopped noticing. As far as gameplay, shooting stuff is probably about the smallest part of X3, though TC seems to lean a bit more towards it. The business side of things is a huge part of the game. Trading, building factory complexes, constructing your fleet, then of course the exploration. And as Bos_hybrid mentioned, there's a great Mod community built around the X games.