Up drive? I think you mean underdrive. I've had the 25% BDR one for a few years. It's ok, has'nt caused any problems or any thing. It's not gonna drasticly make your car faster or any thing though, so dont exspect too much.
I have the 50% UDP and it made a huge difference. The engine revved much much faster. Sorta like the throttle response improvement of a TB... times 3. As for power, the butt dyno isn't terribly accurate......
I haven't had any electrical problems from under-driving my alternator but the A/C is not as cool in the summer from under-driving the compressor.
There are debates that occur on here whether a UDP can harm your engine since the heavier OEM pulley acts as a harmonic dampner (the service manual actually calls the crank pulley a dampner) but there's the camp of people: "I have run one for X years with X thousand miles without problems" and others: "it damages your engine, my friend blew up his motor cuz he used a UDP" etc. It's a milder version of the motor oil debates. Make your own decisions.
For me, I doubt I will notice any adverse affects before I get rid of the car.
i've had a 25% udp for some months now. like jeff said, you wont notice much of a difference power wise. what i did notice when i installed it was that my engine ran much smoother at idle (i was literally like "crap thats smooth") compared to the stock pulley, when i had the cruise control on while driving on the highway i could now climb hills without the car shifting into third gear, and my gas milage went up (i got 645km on 35L of gas once, after the udp)