Shortly after purchasing my first truck, I had it professionally undercoated. There was no rust anywhere, not even on the frame, and I wanted to protect my investment. Not too long after, I rolled the truck (oops) and one of the box sides was replaced and the entire truck was repainted and undercoated. I washed and waxed this truck religiously. Rust bubbles developed after one winter. I touched up the rust as fast as it would form in hopes of stopping it.
After 3 years of winters here, rust formed along the box seams, under the plastic throw-in bedliner, and holes formed around the spotwelds above the rear wheel wells. Those of you familiar with Toyota trucks will know exactly what areas I am referring to. To make matters worse, the "new" box side that was installed as a result of the accident rusted significantly worse than the factory side. How could this be? This truck is undercoated, washed, waxed, and has a fairly new paint job and plenty of touchups. It "should" last a long time!
A poor engineering design is the culprit. The lower quarter panels are spot welded on to the upper box sides. The joint between them is bare metal, and paint cannot get in there. Seam sealer is used to "seal" the joint, but the sealer doesn't cover much. Water, salt, and dirt enter from the open joint on the underside of the box. After a short while, the paint over the sealer cracks and allows contaminants to enter from the visible portion of the seam on the outside of the box. The paint over the sealant cracks due to the normal box flex while driving. The paint cracks are most likely to occur where the sealer ends at the wheel well opening. As for the ugly factory spot welds over the wheel well, there is no paint in between the metal either. The tires fling contaminants straight up into that joint. To make matters worse, though body shop repair quality and paint quality has improved dramatically over the last 10 years, it still does not match the quality of paintwork and welding done at the factory. That and many body shops don't have the standard of ethics they really should.
There have been some other rumors floating around concerning "sabatoge" by the Domestic auto makers on the new import 4x4 trucks. To reduce import taxes, Toyota trucks where built in Japan but shipped to the U.S. without a box. The boxes were built in California and installed in the U.S. I have heard that recycled steel may have been used, and that the poor design was intentional to reduce import truck sales. I cannot verify any of this for you, I am only passing on "rumors" I have heard.
You're probably wondering about strength and looks of fiberglass beds. With both beds I've owned, I have installed a 100 lb. custom Smittybilt stepside roll bar and hauled my fair share of loads including 1,200 lbs worth of capping stones. All fiberglass or galvanized/fiberglass beds I have heard of had a metal floor or a 3/8" plywood sheet and 2x4 crossmember reinforced floor. The modular Stepside Industries bedsides were a little weak and flexed easily. I I fixed that by making reinforcement straps to the fenderwell corners to strengthen the entegrity of the sides. The full fiberglass bed is much stronger and has no flex at all and I did not need any reinforcements.
As far as fitment and looks go, that depends on a few things. Depending on the quality of the mold a manufacturer uses and how talented the builder is, your bed may need varying degrees of bodywork to get that "show truck" quality of straightness. For the most part, all fiberglass parts for any vehicle have some degree of waviness. This can normally be fixed fairly well by the paint shop though. The Stepside Industries bed had too many curves and corners for any kind of waviness to show up unlike a large flat panel. I don't drive the truck much that the new full fiberglass is on and haven't painted it yet (and may have to sell it so I can build a garage)..
I haven't seen any other other beds up close, so I can't comment on any other manufacturer's products. I have spoken to a few of them on the phone and through email. I have spoken with the owner of EFD Inc. (formerly called SVC) located in Lower Michigan once on the phone. He is very difficult to get a hold of as I never actually got ahold of anyone when calling. He was also very slow in returning calls (a week or two). he sent me some pictures and an order sheet, and his prices seemed reasonable. The other company I have dealt with is The Toy Box located in British Columbia, Canada. The owner is Corey Maidment, and I have spoken with him both one the phone and via email. He seemed honest, nice ,and helpful--qualities lacked by many businesses these days. Corey's prices may be listed in Canadian funds, so if you are from the U.S. make sure to convert to American funds for the actual cost.
Here is my contact list of Fiberglass Bed/Alternative Bed Manufacturers. Note that I do not condone any of these companies. I am merely providing contact information for you. If you find any of this contact information incorrect, please contact me so I can update my site.