Onur,
I have the Eisenmann 76mm (round). I've decided that it's the optimum size and falls in between the stock ArvinMeritor 65mm and the Eisenmann 83mm. The 83mm looks too large in my opinion (too ricey!) and the sound of the 76mm and the 83mm are pretty much the same. The 83mm has also perforated resonators inside the tips that I didn't like. The Titanium one is too expensive in my opinion for a road M3.
In terrms of sound, it's much louder than the OEM and makes its presence heard. Especially, it screams under load. Which is not always a good thing in city driving but it's a choice one has to make. To appreciate it, you have to hear it from another car. Also, turns heads more as you drive around. Again, this has pros and cons. Long distance driving gets more tiring after a few hours as well.
One general problem with the Eisenmanns is that they resonate (or what ever you call that pirrr sound) between 2200-3000 RPMs. It's annoying and you want to stay out of those revs. But that's how they are and again a choice one has to make.
Overall, I'm quite happy and I would have made the same choice again for my conditions. On the other hand, the stock
ArvinMeritor exhaust is also a brilliant piece of engineering developed to satisfy several constraints. Read the pdf in the hyperlink.
I have collected sound and video clips of different types of exhausts for the M3. I can email them to you if you like.
Cheers,
Ufuk