Personally, I don't own any of the games myself, but I have a friend who does and he's a big fan.
Basically, X-Wing Trilogy is just a boxed set of games from the series.
If you already have X-Wing Collector's CD, TIE Fighter Collector's CD and XWA, you can save your money.. unless you just want a shiny new box.
It also comes with the XvT demo (14 mission flight school academy), but I'm sure you can just download that (or get it free with a mag or video card someplace), or buy the real thing pretty cheap.
Now if you are like me and don't own the games, it's a real bargain to get them all together in one pack like this. ; )