I had a GAHH top installed last spring after tearing the plastic window during the winter.
It is the best upgrade I've done to the car. I love it.
As for your questions:
I treated the top about a month after buying it; when it was definitely stretched and ready for normal use. I use Raggtop.
I don't know how hard the defrost harness is to install, I haven't done it yet. But looking at the way it goes I don't think it'll be difficult at all.
The visibility out the back is cut in half by the smaller window. But that is a small price to pay for having glass and a defroster, IMO.
