Hit or miss depending on the kit... Some kit sizes are different than the "usual" standard so you'll want to get their bushings. I've received some bushings that were out of round (but I've had some from other suppliers that were as well). The Zen kits are not bad in general but I avoid the gold plating. ALL the chinese made Zen kits have weak magnets with the gold plating. Chrome/Gunmetal seem OK. Slim lines are about the same quality I see everywhere else. Gentleman pens are pretty cheaply plated but are OK if you want to add a budget priced capped pen. The Sierra styled ones don't see much worse than any of the other ones I've seen out there.
Shipping is pretty quick, at least to the US. As long as you buy in enough bulk, there is certainly a savings. Quite a few of the pen supply places are selling chinese kits that are equivalent so unless you're getting the Dayacom stuff the quality isn't going to be much of a variance.