Dave -

I think you're going to find that a cross section of owners here (3.0i, 4.4i, and 4.6is/4.8is owners who went both routes using either straight pipes or X-pipes. Do a search on X-pipes, straight pipes, or resonators and a bunch of forum messages will pop up. Everyone seems to be generally satisfied with what they did, regardless of which mod they picked.

From what I gleaned out of the boards, the straight through pipes are louder, throatier, but have a tendency to burble and pop on decel which some people like and others don't like. They also have a tendency to have a slight pinging or rattling sound when cold, but the sound goes away when the pipes warm up.

The x-pipe increases the sound factor, but not as much as the straight through pipes. The X-pipes provide a small amount of additional back pressure which helps minimize torque loss, but it's mostly negligble. The burbles and pops are minimal or gone completely with the X-pipes.

Horsepower gains are negligble with either solution, although some owners have reported a seat of the pants feel of more HP. However, everyone has reported a definite auditory improvement at high revs, but mostly outside of the car. The increased sound level of either solution doesn't intrude on the sanctity of the passenger compartment.

The exhaust note of the BMW engine can be like angels singing, but unfortunately the BMW designers didn't quite understand that until recently. BMW's current vehicles have a far better sound than before.

$150 parts and labor seems to be a good target price for either solution.

In either case, both solutions are better than the stock setup. It's an owner preference thing. Don't expect any major HP gains with this mod.

(Other members, please correct me if I mis-stated anything here).

Regards, Mike