I've used them for years for web development purposes. All good, eventually. I've got a bunch of folks I use often.
Ooh, just realized I have another project that Fiver might be useful for.
I especially like using the guys that offer weird stuff. They tend to be creative.
For success you do kind of have to know how the place works. Get comfortable with the level of English language your hiree shows. And be prepared to redo any tech stuff, but often it's easier to redo something than it is to do it from scratch.