It really depends on what you're going for. I've had to use a mix of tools to cut it. If you're going "against" the grain, it sometimes rips, which is unfortunate. Luckily with brick, you can chip away at it and it'll just look work.
You may need to "score" the lines with a sharp blade first, then re-trace them with a metal pin or something so they get the right depth, but retain crisp edges.
Like I said, I have varied a lot in my methods with some trial and error, and usually the foam is forgiving enough for the final surface I was going for, but I sometimes have to add a little "damage" to hide some goof-ups. It'll only add character to the building honestly.
I recommend using the foil to go back and create some variance in the texture. The harder you press, too, will create "dents" and other blemishes that make it look more realistic.
Of course, it'll really come to life when you put the first coat of paint on it. That's when my worries finally came to rest, when I saw how good it looked with the paint. That's the real turning point, and I imagine you'll figure that out yourself.