Hi,
As I'm currently a master student at the KUL, let me give you my opinion. I'm in the exact sciences, so I'm not very familiar with the philosophy department. I can tell you about the general university system though.
For Belgian students, there is no admission. You basically sign up for an bachelor's degree (which would be similar to undergrad) and when you're done, you go on to get a master's. There is likewise no admission necessary for the master's. The only requirement is that you've taken the prerequisite courses at undergrad level.
For internationals, there is an admissions committee. I don't think it's too difficult to get in, but I'd check with someone in the international office.
As to exams and grades. Seeing as everyone is admitted, the first year is usually a way of determining who is fit for the degree and who isn't. This doesn't apply for all departments, but in the exact sciences it's not uncommon for about 1/2 of all first-years to drop out of the degree (though this is less for social sciences and other degrees). Also, you have the chance to retake your failed exams in August. And a lot of people do have to retake exams (I think you'll actually find more people retaking them than people who passed everything on their first try.) This gets better as you progress though. If you get through the first year, you'll make it to the end and you're grades usually progressively get better.
Grades are on the average lower than elsewhere I think. You graduate cum laude with a 69% and up, magna cum laude with 77% and up, summa cum laude with 85% and up. The grading is very dependent on the faculty though. There are some where everyone graduates cum laude and some where that's a real challenge. Your exams are graded out of 20. 16's are very good and well above average.
In terms of the city, it's great. During the week you'll see only students and there's a party every night, everywhere. Bars are very cheap and there's a lot of student-run bars where beer is 1€. The only downside is that all Belgians leave to go home for the weekend, so on Saturday and Sunday, it's pretty deserted.
That's about all I can think of.
This is my view. People in other departments may give you other opinions.