I have 2 questions, how common was the HBT uniform in the European theater and were leggings used with them?
Weapons:
King Arms M1928 Thompson Submachine Gun
AGM Sten Mk.II
CYMA M1911 EAP
It was an issue item.
How common depends on the orders given for men to wear it? 
It was primarily a work uniform. Because of this there are plenty of surviving examples in Normandy. Lots and lots.
Why? Who can be sure but my theory is that they were left behind when the troops got out of Dodge. The troops took the hard wearing uniform? Did they take what they were stood up in and leave the work overalls? Who knows but the fact that so many survive shows that for they were issued. How quickly were they replaced? Again who knows, many of the previous owners were killed or captured in the exodus from Normandy.
They were popular in Italy and the South of France for every day wear, not every unit got tropicals.
By leggings do you mean the short gamaschen, canvas with two leather straps, similar to the British ones?
If so, yes, they would be worn with HBT
aka Stigroadie
AFRA
better by design
"Truth is a shining goddess, always veiled, always distant, never wholly approachable, but worthy of all the devotion of which the human spirit is capable. "
When I say leggings I mean the iconic tan ones that you see in a lot of films.
Weapons:
King Arms M1928 Thompson Submachine Gun
AGM Sten Mk.II
CYMA M1911 EAP
Sorry, my mistake. 
I have given you my opinion on the use of HBT by German forces. 
I must read all the headers to a topic.
US combat use of HBT was also not as widespread as some would have you think. Again it was primarily a work uniform.
Several units wore HBT as they assaulted the Normandy beaches, mostly as an anti gas effort. The HBT were treated with a chemical brew before the assault to ward off some of the effects of a gas attack.
Leggings/gaiters were worn with this uniform.
After the assault phase it tends to be the wear of tank crews, mechanics and engineers rather than infantry.
You do see the trousers worn as an extra layer with an M41 rather than the full HBT suit worn as combat wear.
aka Stigroadie
AFRA
better by design
"Truth is a shining goddess, always veiled, always distant, never wholly approachable, but worthy of all the devotion of which the human spirit is capable. "
Right, thank you very much Poacher, what's the usual method of cleaning an M41 Field Jacket?
Weapons:
King Arms M1928 Thompson Submachine Gun
AGM Sten Mk.II
CYMA M1911 EAP
Water, soap, scrubbing brush and elbow grease.
Some repros can go in the washing machine, most, because of the mixed fabric content are dry clean only.
A dirty M41 is not a bad thing, brush off the caked mud when dry and you are good to go.
The only time I wash outer layers is for parades, rare in airsoft, and when the 'smell of man' gets too much even for me.
aka Stigroadie
AFRA
better by design
"Truth is a shining goddess, always veiled, always distant, never wholly approachable, but worthy of all the devotion of which the human spirit is capable. "
