Yes, I have PVC pipe in my tanks. My guys love 'em.
I use the little connector (coupling) parts. I think they're about 1" or 2" in diameter, and maybe 2" long. (They're used for drinking water, so I figured they wouldn't have anything harmful in them.)
I tried the elbow ones, but my guys didn't like the bendy portion. They seem to prefer the straight pieces.
Some people use silicone to attach rocks or gravel onto the PVC, to make them prettier or camoflauge them in the tank. I had trouble getting the gravel to stick though, so I just left them as is.
I use the Loctite Waterproof Silicone
for my projects. I got it at Lowe's for about $4.50. The tube specifically says it's aquarium-safe. Not much is needed, so the tube will last me a really long time. It says to cure for 24 hours - but I always give my projects a couple of days, just to play it safe. The stuff holds up really well. I've even boiled some of my decorations, and the silicone stayed intact.
I've read that some other brands of 100% silicone are fine too. (Someone recently said he's using the GE brand.)
Other projects that I've done, include:
I bought river rocks at a Dollar Store, and soaked them in hot water (letting the water sit overnight), then let them dry. I stacked them and used aquarium-safe silicone to 'cement' them, into swim-throughs or bridges.
I've also used silk plants from places like Micheal's. (They were glitter & fragrance free, with no 'decorations' like fuzzy centers, etc.) I soaked them in hot water (letting the water sit overnight) to make sure they didn't leach dyes, etc. Also, I only used the silk/plastic parts -- making sure there was no metal in the stems.
And I made my own 'hammocks,' using a silk leaf (from one of the above plants) and a silicone suction cup. I used a push pin to create a hole in the back of the suction cup, then pushed the 'stem' of the leaf into it.
I created a swim-through with glass pebbles.
I've read that Legos are safe, and I'm thinking about getting some.
I've got a bunch of small terracotta pots. I boiled them first, then removed the backs from some. and plugged up the holes in others with aquarium-safe silicone, so that the fish can't get stuck. I'd read that they can change the pH, but I tested mine, and the ones that I got do not seem to affect pH.
Some people here use food safe dishware. They posted some pictures of their tanks, which looked really cool.
And someone recommended using plastic drinking cups to create a 'floating log.'
Here's a link to one of the threads where 'DIY decorations' were discussed, and has some nice pictures of peoples' tanks: