For structural purposes, carbon fiber is very very strong. The problem is that when a force is applied for which it cannot handle, it shatters since it's not malleable (instead of giving like metal does). Plus, CF looks awesome

Oh yea, if you break something made of CF, most likely it's done. You'll have to replace the entire thing and cannot fix it to the same way it was before (this was the case with a Ferrari Enzo and McLaren F1).
Fiberglass can produce hairline cracks over time simply because it's not as strong as a material such as CF. It can actually be really strong is you use many layers, but then it gets heavier. Early Vettes are known to have these kind of cracks, but it's easy to repair fiberglass. I say fiberglass is good for looks and affordable (cheap to repair, too).
Uretane and ABS are good for bumpers since they give and go back to their shape with heat (most of the time). It's very light, but I believe it's more expensive than fiberglass.
If you're looking to replace front and rear bumpers, go for urethane or ABS (I don't think they're much of a difference). I would stay away from full CF on the bumpers. I'd use CF for the rear hatch and hood. As far as fiberglass, I wouldn't get it myself. You don't have much of a choice for materials since most companies use fiberglass for bodykits (inexpensive to produce).