I have never had such a domain, so i couldn't tell you anything with regards to traffic but i can say that i do occasionally make this mistake. It's very rare however as i usually do not bother with 'www'.
I see many people get traffic on them - But they are attached to branded names ... and frowned upon for the most part like : wwwgoogle wwwyahoo wwwmyspace etc ...
As far as doing it on Generic words - I wouldn't expect much traffic unless the "keyword" is searched for very often.