I did some sub-contract firm for the bloke who owns the 'Window Cleaning Warehouse' in Chepstow's town centre. He also runs a window cleaning company called Crystal Clear or something like that.
Anyway, I had no problems at all. I felt a little underpaid for the work I did, but he was up-front with the price he was prepared to pay, and paid promptly when I sent in the invoices.
So my experience of doing sub-contract work has been a positive one.
Once I'd built up enough of my own work, I stopped doing the sub-contract stuff, although had I had more of my own work in the area, I may have continued; but our town centre is fairly-well sewn-up by other window cleaners.