I too have fallen foul of flying unwittingly overweight in a PA38, with only 15 or so gallons in the tank. A friend of mine is approx 16 stone and I 12 but I made the mistake of assuming (that word again..) that 2 blokes and only half fuel would be fine and only idly did a calculation to see how much extra fuel I could put in. The result was that my last 3 flights had been 5% over MTOW. At this weight it did not SEEM to make any difference with a healthy (for a PA38) 500ft per minute climb and a seemingly normal take off run. I am in NO WAY advocating flying above MTOW and that was the last flight I made with my friend in a 2 seater. In my case although over MTOW the aeroplane was within C of G limits but that does not make me feel much better to be honest.
One thing that is often said, without much evidence to back it up it seems, is the matter of it automatically invalidating insurance? I can see how it may be an issue if the exceeding of the MTOW was proved to be a causative factor in the accident but I have my reservations (though I am frequently wrong) as to whether it would AUTOMATICALLY invalidate insurance. I would not want to test it though and end up losing all my future earning through being too tight to hire a 4 seater!