I purchased an 07 335i in dec.
I would recommend getting a certified series bmw. I have warranty till 2012.
When I bought this car I test drove it hwy and city and it drove great.
The day I go pick up the car, I get a call telling me the car is not ready.
The turbos had to be replaced.

I was so pissed.
However they provided me with a rental.
I pick up the car and 3 days later during the x-mas holiday the car goes into limp mode. turbos where not working. I took it in and they had to fix some hose that fed the turbos???
1 month later my spark plug light came on. they had to replace that.
a few days after that I noticed the car had the hiccups. The car was not idling properly. They did a diagnostics and couldn't find anything wrong with the car. they told me to use shell v power fuel. I tried that for 3 weeks and the problem didn't go away. I went back a few times and they couldn't figure out the problem. Finally about 2 months of this back and fourth. The car was showing signs of hpfp failure, the car was having long crank issues. I took the car to another dealership, and they replaced my hpfp.
I was so angry with bmw for the first 5 months of owning this car. I'm glad I have the warranty. Sometimes you get a lemon.

But with the warranty they will fix this problems and you will be on your way.
I don't want to discourage you from buying this car. I think I just got a really bad car from the factory. But when you are paying 40k for a car,
you don't want to turn around and sell it. you will take a 5k hit on it. (taxes)
So in my experience warranty is very important.
I am now in the process of modding the car, hopefully with new turbos, spark plugs and a new HPFP I won't have any major issues.