I bought an E89 23i last summer and to be completely honest, there have been a few issues, but nothing serious and I am more happy with it now then when I first got it! Every day I walk outside, see it, unlock it and get into it it puts a smile on my face!
Issues were:
Rear nearside indicator stopped working due to water ingress - known problem on the older units and the whole unit needs to be replaced, but simple job and new unit is a slightly different design.
Small oil leak from valve cover gasket - seems to be common with a lot of BMW engines as the block is tilted over. Again, easy enough to fix.
That is all the issues I have had so far and the car is nearly at 60k miles.
It sounds like your mechanic was making a general comment about the brand based on some pre conceived notion....but not specifically Z4's?