One of the downsides of doing business with China (eBay or otherwise) during a build season is that you can spend a great deal of time awaiting delivery. I've had packages delayed by as much as 82 days. So I might try to find someone in my country that already has these items such that I can get them faster. I do this with other items like Olimex development boards.
I bought some of the SR-04s while I was buying some cheap logic analyzers from China. I find that the MaxBotix unit from a few years ago may be more expensive but it tends to be more consistent across lots. However even the MaxBotix unit could read wildly wrong during periods of movement especially in the presence of noise sources or electrical noise.
Before anyone demands proof that Maxbotix sensors can have issues let me take care of that for you:
http://forum.arduino.cc/index.php/topic,140936.0.html
I think it greatly matters to me what I am using the sensor for.
If I am just tinkering then the SR-04 is generally fine unless there is danger involved (robotic lawnmower for example).
I might be willing to use it on a FIRST robot but honestly I think these sorts of things are like encoders: time is short and you really do not want to be messing around with inconsistent COTS parts.