I'm with the crowd that says using steak-umm or any of that sort of product is cheesesteak blasphemy! However, if you are going to use that frozen sliced beef product, the key is to break it up before you fry it up or chop it up in the frying pan as it's cooking. Don't keep the slices whole.
And really...a real Philly cheesesteak is a roll, steak (get a butcher to chip some rib eye) and cheese (whatever your preference but a true Philly cheesesteak is with whiz). Fried onions are acceptable.
Other than that...I think Jagafen hit the nail on the head...BLASPHEMY!!
Think I'll go get one right now. Saturday's are always hoagies or cheesesteaks for lunch in our household.
Speaking of hoagies. Thats another thing that you can't get anywhere else but in Philly. You can try and duplicate it but it wont be the same.