I travel a lot and most airlines have a 50lb limit on bags. I always have a heavy bag (lots of files, etc.. in addition to personal stuff).
I never used to have a problem, but in the last 6 months or so I have had to pay for overweight bags a few times, on different airlines. It depends on the airline and the person checking you in, but they do seem to be more concerned about it now than previously.
The last few trips I have checked two small bags rather than one larger bag. It's a little harder to deal with getting too and from the airport, I always have my laptop too, but it's better than the fees (I have paid anywhere from $25 - $60) for the overweight bag.