underdrive pulley works very well..it should give you about 1-3 whp which is a pretty fair amount. there are problems with the alternator producing enough electricity though i've heard.
the unorthodox pulleys are also very good..they offer the ultra s crank pulley and the ultra ss pulley set. don't read too much into how they'll add 'up to 15 whp' because that's simply not going to happen with the celica engine. a few people have reported problems with these pulleys ranging from the minor (lost power) to major (my car broke). all in all you can say that it obviously didnt break anyone's car--but the pulleys probably didn't help. anyway, i havn't heard any actual complaints on these pulleys in FOREVER--i'd say they're pretty safe.
Yeah, like someone else said in this thread, you have to remove the harmonic balancer to install the Unorthodox pullies. The harmonic balancer reduced crankshaft vibration, which tends to happen mostly at high RPMs.
Since our cars rev high, it's highly recommended not to install it. It's meant more for muscle cars.
Also, I believe that there was someone who blew their engine somehow with the pulley.