You've got to tell us what you're goals are for us to tell you whether they're worth it or not. A lot of things that you can do on parallettes you can do on the floor, and often this is going to have the same or greater benefits of doing it on paralletes. Sometimes they give you a different option for exercises that can stress the body differently or make a movement easier. There are some things they'd you'd only be able to do on paralletes, so if you're interested in specific moves, then they may be a great option for you.
I like mine for doing frogtands and L-sits on, because they take some of the pressure off the wrists.
Can't you just use those cheap push up stands? Does it make a difference? I know they're shorter but...? Like so:
Think about trying to do an L-Sit on something that low, lol. You might as well just push up on your hands, so you better have great form and control from the start! I've never heard of parallettes before. They look like the middle man for exercises you can't do with the normal push up bars and a full set of dip bars is just too much. That PVC bit Seiken posted looks a bit shifty to me, needs to be metal or 2" PVC for load bearing (for me anyways). @Fusen, it'll be worth it in my opinion if you use them like Frodo does.
I got them as a bit of a try out / bridge to using parallettes. They are also useful for gathering dust in a corner of a room. As for the crossfit ones. They don't look too sturdy, but PVC is often suggested.