Reputable UDP?
 

I saw this ad and for a very reasonable price. Anyone have first hand experience with this ?Porsche 996 / Boxster / Cayman Under Drive Pulley (With AC) | eBay

I have an RSS UDP good quality well made, the one you have looked at looks far too fancy for it's own good, style over substance

This is my opinion based on my research and my personal experience:
An underdrive pulley is vey useful for those of us who drive their cars on the track where 90% of the time the motor is in the 4000 rpm and up range. It helps us avoid cooking the peripherals (a/c, power steering, alternator). It does not increase the horsepower or the torque. If you disagree, show me the proof. If you are not tracking, put your money elsewhere.
Oh, btw, the rss is a good choice.

I have the RSS, I like it, felt a little boost, if I were somehow to get another Boxster for street use, I wouldn't do it again, it was a hassle to install :) on a track boxster, I would probably do it again, mostly for power steering protection :cheers:

If you do some interweb searching, there are a few posts regarding spoked UDP's failing or cracking at the spokes:

Underdrive Pulley Alert

http://www.planet-9.com/987-cayman-and-boxster-competition/74768-bbi-underdrive-pulley-alert.html

I also have the RSS UDP.

I found mine thru Woody. Happy with it.

Just my opinion - the UDP doesn't give the engine any more power but it creates less parasitic drag from the accessories and that frees up a handful of HP for the engine to use in turning the wheels.

Two items I've noted - my engine runs a tiny bit rougher at idle with the UDP and I wouldn't use the UDP to time the engine - keep the original handy for that task.