I've only bought a couple of things from NPD and they were just fine. I've also found they carry some small parts that other suppliers didn't. This topic is a clear example of that. They had the cap decal, none of the others I checked did.
However I found CJs website much easier to follow when I first got my S code, and was still finding my way around Mustangs, they gave me some small discounts after a few orders, and I liked their little customer extras like how to videos etc., even though in reality I didn't need them. However as I say, their advice and descriptions aren't IMHO reliable. I have a decent sized order about to ship from them, but it may be the last like that.
But since NPD sent me one of their excellent paper catalogues I've decided to give them more of my business. They do seem to have the parts I want at good prices, it's just I couldn't find them so easily on line as on paper.
I find it helps if you have the exact PN to put in their search engine, rather than a description. And I've found that even a description that's fairly close to the product I want often doesn't get hits in their search engine. That's probably why I shyed away from them at the start. Possibly something they could improve on...
Mustang depot and CJ's have got upwards of 20k worth of sales from me in the last year or so, purely because they are for me at least, easier to webshop.