There are many reports of issues with newer cameras. If you have a 5D MkIII then don't get one until they are advertised as working with your camera.
Personally, as a budget option I would recommend the Canon 550EX, full functionality with every EOS camera past, present and future, for around $125.
pause and check the website: they are advertized working with the 5D3 and many others
Wireless TTL Slave mode: Canon 5DIII, 7D, 60D, 600D, 650D, Yongnuo ST-E2, Canon 580EX II, Nikon SB-900, SB-800, SB-700, Commander SU-800, Camera with CLS i-TTL system:D4, D3x, D3s, D3, D2x, D700, D300s, D300, D200, D7000, D90, D80
Hot shoe TTL mode: Canon 1Dx, 1Ds series, 1D series, 5DIII, 5DII, 5D, 7D, 650D/T4i, 600D/T3i, 550D/T2i, 500D/T1i, 450D/Xsi, 400D/Xti, 1100D, 1000D, 60D, 50D, 40D
Not compatible with 20D, 30D"
Part of my experience was landing on a reputable supplier. I wouldn't trust any of the ebay stores (except for thephotogadget. read the reviews), nor would I trust the stores selling on amazon. Order direct from thetphotogadget. Avoid all the others becasue you won't know what inventory they've been storing up, and you might get one with the old "1/250th shutter" problem and no way to return it. ThePhotogadget will accept returns, although its true you have to pay shipping.