Electronic Cigarette in Brussels

Not really, you can buy cheap disposable ones over the counter at Relays and such thing but essentially they are banned from sale here, or at least have been.
If you want a good one you will have to go online.

Do they taste like real cigs?

L2 is misinformed. You CAN buy them in Belgium. In a shop in Galerie Louise. Smoky Club I think its called. The brand is called eGo-T - look you can read reviews online. And yes, they do 'taste' and simulate the real smoking experience. Quite extraordinary really!