Ive used PVC but just got to know what youre up against. Custom shapes, temporary? it's good... Putting it outside is a bit of a challenge. If it's a temp sign, YOu have better options, such as Coroplast which Id personally rather use. However if it's a 6" or 12" wide strip, or just in general, a smaller sign, it should be ok, just dont screw the **** out of it when you install it, or it'll buckle like crazy in the sun. I had those letter tracks Id slip a strip it, short life promotional stuff. The colors in PVC will start bleaching overtime, but this happen with Coroplast too.
With the thicker PVCs, you're getting into some pricier stuff. If you have a square sign, go with something like MDO, Alumilite, Signbond, something along those lines. The price isn't too much more, and if you're looking for longevity, I wouldn't bother with PVC ever. It's good for indoor use, custom shapes, plus nicer edges/surface then coroplast.
If your doing a sign against a building, you can getaway with 1/2" ply, or a thinner signbond sheet. Alumilite was one of my customer favorites. It's light, sturdy, and had a nice surface. Can be double sided and is rigid so wouldn't buckle in the heat of the summer, and took a good beating too. I installed 50 at a baseball field, and they took some harsh blows, but just a minor ding. Did them back in 2003, they still are using those signs. 9years later now.
I've been using 12mm pvc for exterior use dimensional letters for years.. Have had great results with stud-mounting them to monuments, walls, etc.
Very important to seal any cut edges by brushing or soaking the product in pvc cleaner before painting, as it closes any open cells left from cutting, and prevents water or algae from permeating the substrate.
The thin stuff is useless, as far as I'm concerned.