I've seen mixed reviews on them. Personally, I'm a believer in you get what you pay for.. I was skeptical on leds until I saw a few grows on em. This is why I invested in a Kind LED K5 series.. figured I'd go balls to the wall with it lol.
I'm running 2 viparspectra 600 watt led light set ups, I've been running hps/MH lights for the last 10 years and I wanted to see what these were about. Not too shabby for the price, and my power bill is about 120 a month so I feel like I'm winning. Im thinking about tossing a 600 watt hps in the middle of the 2 LEDS to see what that does.
I'd say theoretical maximum yield with these is 0.7 grams/watt, when you can max out 1g/w with HID's, and 1.5-2g/w with CMH or advanced LED lights like quantum boards or COB lightning.
DIY LED kit's can be cheaper then a viparspectra as well.
If you are a beginner, and are not sure if you will continue the hobby, Vipar's will do a decent job to get you in the game, and they produce excellent tasting buds, even though the yield might be lacking.
I've used, HPS 600w, Viparspectra 900w, and am now running "Quantum Boards".