Never tried any prefilled 510 cartos, all blank on that front, filled with liquid myself.
I'll second the V4L, while they have higher-than-average shipping, they do offer a pretty good selection of flavors. Def get in on a discount code if you try to order with them (from their newsletter is the easiest). It's easy one-stop-shopping if you are a committed 808 user. Case in point: My dad is in his 60s and about all he can handle with this "fake cig thingy" is cartos. I'd never expect him to refill anything, but a good supply of cartos works. Then I can refill for him later...
Only other place I've gotten prefilled was with my V2 starter kit, which were OK. I wouldn't send anybody there to order for the flavors though.