No, no it's not.  One of the first things you learn in statistics is that you need a sufficiently large, random sample in order to be able to make conclusions based on data.  A few anecdotes is neither a sufficiently large sample, nor is it a random one.

Second, you do realize cars have to go to mechanics on a regular basis, right?  There's this wonderful thing called servicing one's car.  It's where you have the oil and any other necessary fluids changed, you get the items that wear (e.g. tires, brake pads, etc.) inspected, and a host of other things looked over.  Of course, most of these things can be done yourself, but most people don't know how.  As a result, cars are taken to mechanics regularly, a process that's better known as maintenance.  Magic, I know.  Your mechanic seeing lots of Z3s or Z4s doesn't mean anything.

I thought you were bright enough to understand that I meant he was seeing a bunch of Zs on top of regular maintenance, I guess I was wrong. So exactly how many people saying their Zs break down foten will it take you to realize BMWs are not the same anymore?

Well, in a statistical sense, you'd never be able to show me enough, because they wouldn't be randomly selected.  Unless a sample is random, it doesn't tell us anything in statistics and you'll never be able to show me enough.

In another sense, do you realize how biased such findings would be?  I assume you're going to ask your mechanic friend about them, in which case OF COURSE every one of them will have had issues.  They went to him only because they needed something to fix them.  Basically, a prerequisite for a Z in your sample is that it has broken down, because your mechanic friend wouldn't have come into contact with that car unless there was something wrong.

Second, unless you can pull reliability numbers for all other similarly priced roadsters, your results wouldn't mean anything.  Reliability is relative, and unless you have another car to compare the Z3 and Z4 to, you won't be able to conclude anything meaningful.

More generally, I'm not going to believe you no matter how many you show me for several reasons:

1) I've done a lot of research about the Z3 and the Z4 because I was considering picking up one or the other for a while (still considering a Z4 M at the moment), and the general consensus (among countless forums and reviews) is that they're rock solid cars as long as they're maintained properly.  The Z3 did have some issues with the rear subframe, but it was only a problem on Z3 M's because of the increased power, and it really only became an issue if the car was tracked.

2) I've talked to the guy in charge of testing the Z4's, and he gave every impression they were rock solid as well.  In fact, I'm going to interview to work under him on Tuesday.

3) You clearly don't know much about cars, whereas they're my greatest passion.  Believing you over what I've established for myself over the past few years through research and first hand experience is stupid.