Are fiber glass enclosers for subs good?
whats the pros and cons to them over mdf boxes

 

If you build the fiberglass thick enough without air bubbles they look better than wood boxes. They also allow you to form the enclosure to the area. Gives more space. I guess there are a lot of pros if you know what your doing.

 

main advantages are cosmetic. you can mold and form all sorts of shapes with glass. But it does take some skill to glass a box properly. Your first few will probably end up weighing more than an MDF box due to excess resin as you build trying to hide/fix/cover your mistakes. MDF boxes are robust and easy to build but for the most part they are just a box.
Both materials work equally well MDF is just easier to work with.
really a matter of aesthetics, form over function or function over form.