I think the can is very effective! I had my best buck come running in on a string not 45 seconds after I used the can. So it can definately work in the right situation.
The devil in the detail is finding that right situation.
If the truth were told I've probably used the can over a 200 times in the field and only had three or four encounters that I felt like were directly related to the calling. The trick is that those 4 encounters ended up with me killing three nice bucks so it was definately worth it. I can't speak to any evidence of them spooking deer. Just like grunting they often don't come running over if they are moving away from your position and you try to use it to turn them back.
Here are the situations where I think they can help.
I had a doe run up behind my stand in obvious chase mode. The buck was not within sight. She turn before ever coming within shooting range and left in a hurry. I started calling with the can as soon as she left and the buck just happened to come along about 30 seconds later. He heard the can and came directly to my stand looking for the doe he had just been chasing. Dead buck!
Another time it worked was last fall after I had done a series of calls on the can. I had a doe slowly filter out of the woods with a buck calmly following her in a tending fashion. I think she wasn't quite ready or was already bred but just uncomfortable having the buck around. She wasn't running to escape the buck but just came to the sound of another deer. In doing so lead him right by my stand.
Chris