I have bought a 901 and some liquid from Jane at e-cig sales, replacement parts and USB charger from Katy at the Smokeless shop,
eliquid and a few USB passthrough batteries from Steve at Puresmoker, and a David Yang penstyle from Gashin. All of these have been quality parts and good liquid. Customer service has been phenomenal, shipping reasonable, and no communication problems. All of these are suppliers listed in the Suppliers list.
Buying from the suppliers who post on this forum, you will probably not find a strong difference in quality. They pretty much all supply the highest quality product they could find, perhaps many from the same wholesaler. It does not matter to me, really, if they come from the same wholesaler or different ones. I really just care about the above stated things - quality, price, communication, service, and timeliness.
If you want to see quality differences, try ordering from suppliers who don't post here. Two were mentioned earlier in this thread. You can get product cheaper from them, but have fun trying to get any service when you get your low quality stuff. Our suppliers are really the guys who sort out all those details for us. They figure out which wholesalers have good product for good price. They search and find the best stuff out there. They warranty the product, thus saving us the hassle of trying to get any service from the wholesalers who speak only a very broken English and that is only if you somehow manage to find a raw nerve that gets them to answer their emails or their telephones.