Lets just look at the post.
"HE was usefull vs tanks, not as good as AP vs thick armor"
No major objection to this -- AP is better than HE vs. thick armor. The word "useful" could be objected to by some, but I suppose HE could be useful if you had nothing else handy. You will have almost no chance to actually kill a tank with it, but taking out a track with a good hit can happen. Plus the big HE rounds can create spalling (which is where the shockwave from an exterior hit causes flakes of metal to break off on the inside and kill crew).
http://en.wikipedia.org/wiki/High_explosive_squash_head
"but vs med armor they were great ( from big caliber guns)"
The word "great" is what people object to since it implies that it was something people would happily use in that situation. This is inaccurate for WW2 era equipment.
A HE round is basically a big explosive surrounded by something meant to shatter (shrapnel). It is used to kill soft and semi-soft targets. Imagine hitting a tank with a big hammer -- that is a HE round (the shrapnel effects are inconsequential in anything but the lightest of armors). If the hammer hits hard enough it would kill (or more likely damage) a tank, but it is far from a "great" weapon against them. Instead you fire at the tracks and hope for a lucky hit to kill the track.