Guys, feel for you, I don't have such issues with mine, it has proven to be worth every penny for me and a couple of other forum members I have helped, so has paid back already.
It is never going to replace a dedicated system, and some things are gimmicks. That said,the other thing to bear in mind is that the Z3 is an old car, and it is limited on the number of modules that can be read on any system, but for engine diagnostics the elm device is adequate.
It's also worth noting that no diagnostic system can tell you what is wrong, but only tell you where the fault is coming from, you then have to do some leg work to examine that system to prove which part is at fault.