Tricky sentence

Better yet:

"This device - as you are probably aware of from using it for some time, is one of the most difficult devices for employees to use properly."

Your sentence is correct, but "as you probably know if you have been using it for some time" - I feel as if it can be improved via vocabulary, and "is one of the hardest devices"...I don't like the word "hardest" because even though it indicates a level of difficulty, I believe that "most difficult" would be better suitable to what you're trying to stay to your audience. 

All in all, you have the right idea - but I would just improve the vocabulary a little.