I've just bought into Gelish and I love love love the product. It's been a big investment for me as a fairly new nail tech and is proving popular with my clients only a month in. I wouldn't consider changing to another brand just for selling in Sally's until I was definitely sure that it was having a detrimental effect on my business, I haven't got the capital for a start!
I doubt very much whether many people would invest in top it off, foundation, cleanse, remover, lamp and a selection of colours. Those that do want to try it out themselves will probably buy knock off products from Ebay for half the price then wonder why they get awful results.
I know I'm just re-iterating what others are saying, but wanted to put in my twopenneth worth
. I think though if 'joe public' want to buy it badly enough they'll find a way, Sally's or not.