As an American, I was exposed to the CFL while in college in Buffalo, New York, and I stayed with it because it was football, it was fun to watch, and because some of the players were Americans who recently graduated from college in the US and were able to persue football in the CFL after being cut by the NFL.
The problem is that at least in the Philadelphia area where I live, they did show one CFL game per week on Sunday afternoon (a rebroadcast from the previous evening), but sopped as soon as the NFL season started on September 11, 2005. It's the old "Out of site, out of mind" situation. If you ask me which I prefer, there is no question I'd say the NFL because that's what I was brought up on. I do enjoy the CFL however, and the CFL has some stars that could easily play in the NFL and be major contributors to their team's success.
In the US, if you get away from the US-Canadian border, there are many people that have never heard of the CFL, wouldn't know football is even played in Canada, and couldn't name three teams in the CFL. I think they are missing out, but I also think the CFL could cultivate fans close to the border, but would fail miserably if they went far away from the border, because the interest wouldn't be there. It's the same with buying something in Canada with American money, everyone in Canada knows the current exchange rate, while if you try to buy something in the US with Canadian money, they would know the current exchange rate close to the border, but if you were in Atlanta for example, they wouldn't know what to do, because they almost never experience it.
I think the CFL is an exciting brand of football, but it would have limited success in the US, especially far from the US-Canadian border.