The metro system in Brussels is the way to get around. It goes out to the end of the city limits and most of the time you won’t need to go that far. Fare passes are 1.5 euros a ride (I believe you may want to check on the prices), they sell them in single rides, five rides, 10 rides, and even monthly (these are also the same passes for the bus and tram). If you get the monthly pass, then you will have to take a picture to go along with the pass.
I would strongly suggest riding the metro in Brussels; I have never experienced a problem with it at all. I rode the metro every day to work, to school, in the morning, afternoon, evening, late night, and early the next morning. I have never felt unsafe in the metro and feel like it is the easiest way to get around. In fact, you can take it to the train stations, in case you are continuing your journey onward. You can’t beat the price (it's cheaper than a cab), and most of the time you will have a seat to sit in.
The metro stations and the metros themselves are clean, respectable, and do not give off that aura of crime that they sometimes do in other countries. Every station has a different theme, and they are decorated by Belgian artists to reflect this theme in everything from the walls to the chairs that are provided for you to sit on while you wait (a nice touch). Another nice touch is the fact that the metro stations all have a little sign that will tell you where each train is and how many minutes it will take the train to get to you.
Several of the larger stations have stores, restaurants, chocolate shops (check out the Leonidas), shoe shines, and everything that will make your day easier. A lot of these places are closed on Sunday, so be aware of that. But other than Sunday, you can stop and get a croissant or a newspaper to read, or perhaps you're in the mood for a knock-off purse. Several of the stations have these, as well.
Every state was clean and had at least one attendant there in case there was a problem. I was there once and had to fetch the attendant to help a person who collapsed. They can also help you with directions, and most speak English.
There are only two lines to the Brussels metro, so you can’t get too lost. And if you do get lost, there are maps posted everywhere. My only complaint is that the metro closes too early (well, at least while I was in college). It closes at 11pm on the weekdays and midnight on the weekend, but then it does start up again at 5am. So you could do what we did a few times and take the last metro into the city and the first metro out.