I've had the Easy Fermenter kit for about two months now and I've been very happy with their build quality and their effectiveness. They seal very well on my wide mouth mason jars and the airlock seems to work perfectly. I've done 5 different ferments and have yet to encounter any problems (other that a few false starts during my initial ferments that were entirely due to my inexperience.) I did not leave enough head space when filling the jar the first time I made pickled red onions and had liquid exiting the airlock which had the side effect of causing my kitchen to become overwhelmingly oniony (to the point where my eyes were watering). The final product was still delicious.
Have not had any problems with mold or contamination, the instructions were simple, clear and easy to follow and their follow up support (forums, newsletter, videos) have been very useful.
The only thing I've noticed is that the O ring can pick up mild odors (the lid I used for jalapenos smells pleasantly spicy) but it doesn't appear to affect the flavor of subsequent ferments. If you're worried you can dedicate a particular lid for strongly scented ferments like peppers or onions and use a different one for other veggies.
I would recommend investing in a set of pickling weights as it's an easy way to reduce the chance of mold.