If there is a downside to this wonder material, it's that it seen as a red flag of cheap production values. No matter what the final image acheived by using a PVC rig, there is a good chance you'll raise the eyebrow of disapproval from the client base. This doesn't necessarily mean they are snobs, but that in general, white plastic pipes don't scream high quality gear to anyone. It's cool what you can achieve with this stuff on a private level, but if you choose to make money, presentation is part of getting work.
Kind of a nifty movement that raises the bar of DIY quality is the use of metal flash brackets to make some pretty cool and professional looking rigs. These are made from hard aluminum with a nice black finish. They come with rubber padding and 1/4-20" knurled knobs. Made to hold cameras or flashes in various configurations, they can also be put together to make just about anything. Though more expensive and not quite as versatile, they look great and add a touch of class if you are willing to spend just a little more for it.
Dual L Brackets - $10
Single Flash Bracket - $5
This is a very simple aluminum piece with a channel and knob setup that has a cold shoe mount on the far end. I've never liked the cheap shoe mount (and always replace them), but this little thing can be very useful when modified. This was the basis of my Frugal Camera Fattener which gives my small camera a larger area to hold when shooting and I use it all the time.
Dual Straight Bracket - $8
This is a straight aluminum piece that has a 1/4-20" thread flanked by two channels with matching knobs that you can attach two items to. Chung Dha created a "camera cage" with two of these and some hand grips and has made all kinds of DIY stuff from these brackets. Be sure to check out his YouTube channel.
Foam Camera Handle - $7
Camera handle grips with a 1/4-20" thread (male version on one side, female on the other) will come in very handy when you are making any kind of rig based on these brackets. They screw right in to any number of points and are cheap enough that you can get quite a few for not much money. You can always make your own, of course, but these are pretty decent (and decent looking) for an inexpensive retail product.
C-Bracket - $8
This one is a little more unique and less used in the DIY world, but it's out there and you might be able to work it into something you are wanting to bring to the world. I've been trying for awhile to re-work the Frugal Floater using this bracket (sadly, with no luck) and Chung Dha has used one for his Gunstock Camera Rig.
So there you go. If you are considering designing a custom rig and you care about how it looks, you might want to check out some of these brackets to see if they will work for you. They do cost more than PVC, but they look great and are stronger than plastic (which means they should work better with larger, heavier cameras). I've been using them a lot lately and am very pleased with the results.
One final note about these brackets. PVC pipe and certain parts may be hard to come by depending on what state or country you live in. All of these above mentioned parts can be found through many vendors on eBay, making them equally available to just about everyone.