I just sold my gl2 but i had an ewa housing and it worked great. i also have one for my Z1. Don't compare them to a ziplock bag, i haven't had any issues with either. The one good thing is that you have acsess to every button. So unless you are going to be doing some deep see diving i would recomend it.
I have and epic housing and a EWA housing. The EWA is much better for weight, and not getting bonked in the head on the turnaround.... I have more problems with the Epic leaking around the clamp too... I'm working with them to get a new clamp but if you over tighten the clamp even a small amount it kinks and mine leaks. I love the Epic for surfing and playing in the pool... it helps me float... both are really nice and I use GL2's in both. Neither are rated for underwater stuff beyond a few meters.