I own the Waida Silver model and it is a very nice mod, the build quality is superb, for the money you pay it is very nice and performs as expected, TP working well with nickel builds and the fixed 510 I have had no problems with, everything I've thrown on it works.
The Waida is generally accepted as the better made flask, they apparently all use the same chip, but the Waida is more 1:1 with the original VF. Waida also uses a cradle for the insides opposed to just hot glue.
There is a somewhat dodgy grounding technique used for the Waida which can cause some problems but is an easy fix, mine has no problems but I've read of others with it. Saying all that the Kangxin is a solid perfromer from most accounts from what I have read, also some reports of Kangxins dying also, a lot more than the Waida flasks it seems so far.
They are both nice flasks but I'd go with the Waida, I have read every page in the VF clone thread and they both have pros and cons so I don't think you can really go wrong with either.