I do have asthma. On occasion, it does get bad enough that I feel like I'm about to pass out and nearly have.  I can however, be quite active. It just depends on the air, my allergies and my overall mood.
In the winter, as soon as I step outside, my chest tightens and I have a difficult time breathing. In the summer it isn't as bad, unless it's extremely humid out.
I only have a severe attack maybe once every two to three months. It gets so bad, and no matter how deep of a breath I try to take, it just doesn't do any good. My lung capacity feels like it's nothing and my chest gets incredibly tight. I find it's a matter of trying to stay calm, which is harder than it seems. I start to panic, and it only gets worse. Not really my idea of fun...
Of course sometimes an attack can be brought on by nothing, which is why I have to take my inhaler everyday.