I'm in the motor trade and we fix all makes of vechicles and use aftermarket stuff..expensive cars too with no problems..these parts have to be made to a high standard now a days.
But with my own car i use ford stuff but thats just me been doing that for years even with my vauxhalls.
If your going aftermarket than ask for the better stuff like FRAM etc and not the dead cheap bits.
You can get a full major genuine service kit from carpartsexpress for 85 quid and castrol magnetic for 33 quid. Or you can get non ford parts for what say 30 quid less? For 30 quid just buy the right stuff.
The majority of service parts are all made in the same factory, just with different labels and put in a different box. The only reason I use genuine is because I get a lot of things at trade price and I like to keep my history file full of Ford receipts.
On my daily mondeo I use cheap service parts from either a motor factor or ebay, they are always the same quality as parts I would buy over the Ford counter and I have never had an issue. Having said that, for something like a cambelt kit, I would go genuine as if it were to fail, you have somewhere to point the finger!