Some trains are mobbed, but you'll be able to find a seat on most of any. There is no extra charge for InterCity trains. I noticed that most of the newer stock was centered around Brussels, as is all too typical. Belgium seems to be in the process of upgrading their railways a bit at the moment. The seating is not disagreeable.
Trains run quite often.
I'd like to say that they're on time, too, but unfortunately, about a third of the trains that I took when I was there recently were fairly late. One was where we were changing trains in Brugge. However, the connection so late that a local train replaced it as the connection. This may just have been due to the fact that the train we were supposed to catch was an EuroCity, which are, by definition, late. All in all, it wasn't too much of a wait because of the plethora of local trains.