I really want a Eisenmann quad exhaust system. Really. After looking into adding an after-market exhaust that wasn't Dinan, to my Dinan modified 335i, I decided to wait until I'm out of warranty before I change the whole exhaust system on my car. A good modification that is low cost and adds a nice after-market look is exhaust tips.
I went with 3.8" chrome tips from Cyba off the E90 board. Ordering was easy and the price was far better than Dimsa tips. The Cyba tips ran $99 including shipping. Dimsa tips are $145 (special right now down from $175) not including shipping.
Locally, I went to S & K Muffler in Royal Oak, Michigan to have the tips installed. They do require that the stock exhaust is cut and the tips are welded on. Cost was $60 for the installation. It took about an hour to install, while I waited.