Well, I bought some and must say I am fairly impressed by the quality given the price.
The kit I got came with rubber spring washers to eliminate squeaks and rubber o-rings to ensure a tight fit to the shock body.
I have a set of Ground Control coilovers on my Miata racecar and the quality of those is outstanding, but the China made version isn't too bad in comparison.
PROS:
They are cheap!
Machined parts fit stock shock shaft perfectly.
"Universal" collars fit shock body reasonably well and stay tight.
Spring rates provide a pretty good handling balance.
Ride height is adjustable to your specific taste.
Accepts standard 2.5" race spring so you can fine tune the rates by using Eibach ERS or Hypercoil springs (about $80/pair)
If the spring does sag, just adjust the coilover up to compensate
CONS:
Spring rate is high (450F / 350R), about 3-4 times stiffer than stock and about double the rate of the Eibach Pro-kit. Not bad on the street if you like a firm ride. The only problem I found is getting a shock stiff enough to handle the spring rate. Tokico Illumina is the best I could come up with short of a special valve Koni race shock. I am pretty picky with my bump and rebound settings so the Tokico set-up is only okay for me, but someone less critical may think they are great.
The Tokico has a ring welded on top during assembly that must be ground down slightly to fit the coilovers. Easy if you have a bench grinder/sander. (While you are at it, elongate the lower mounting holes for a cheap camber adjustment!)
If you retain the stock spring perch, only lowers the car 1" +/-
Several kits are out there, make sure you get one with the rubber washers and tools to tighten the collars. I got mine from RedDog Motorsports (Ebay) for $70.
They are a little more work, but if you are good with your hands or have a good mechanic it is worth it. When done, I will have mine set up with the ride height I like (lowered look but plenty of suspension travel for fast driving) and the neutral handling balance I like. It will run circles around a car with a stock or basic Eibach kit for less money. At least I hope...
Good Luck!