Underdrive pulleys??

Which underdrive pulleys are better (in terms of keeping harmonic balance, giving the most hp gains, and most of all not screwing my engine) Or should i just not bother with them?

i've already decided not to bother with it, because i heard of a guy whose engin needed all new berings and the crank fixed after just 50K miles, the last 15k of it was with the under drive pulley, plus i know everyone has heard it is good and bad, since i need my car every day im not even going to bother with it, even though i know someone who saw an incredible difference after the pulley :/