I have a flexiglass challenge (was on it when I bought the car). It is a piece of crap. The locks are buggered, it leaks like a seive, the bolts they put in all rusted and came loose, has to cut them off cause couldn't get the nuts off, it has cracks in the external fibreglass, the brake light is the cheapest thing I have ever come accross, the mounts for the rear window gas struts bend......... Apart from that its pretty good! I do however like the tilt up side windows and internal frame for roof rack monting.
Make sure yours has
1) Good mounting system
2) Internal frame
3) Interior light
4) Tilt up side windows
5) The extra high ones are practical but look a bit funny, i'm not sure on that one.
6) Good seals.