do you need one?

depends, do you live in a place where electricity is not clean anymore? (a large appartement, or similar)? or do you have huge power flucuations? if not, you'll still do pretty well without...

They can make a big improvement, but at the same time, there also lies a 'danger' within them. Since what you want in a power conditioner is that it 'cleans' the power, which means it's going have to filter the power. This can limit the dynamics in your system, 2 good brands I know are PS audio (especially the regenerators) and Isotek.

unfortunately, they're both pretty expensive...

Keep them spinning,
Bert.