Sarah Bird at SEOMoz has an excellent post up on the differences between ACPA and UDRP. She touches on a few of the related issues of why someone might want to bring an ACPA suit instead of filing a UDRP.
If someone owns a domain that you feel infringes on your trademarks, I highly recommend reading the post in its entirety.
The one thing that she misses that I feel is worth adding to the discussion is that most ACPA plaintiffs will ask for an injunction against transfer as they simultaneously file suit. This prevents the domain owner from transferring the domain to a registrant outside of the US (where ACPA cannot recover the domain).
Some domain lawyers recommend that you file the ACPA even before you send a Cease & Desist so that you can lock the name and prevent its transfer.