Speed on file downloads or streaming audio/video is not improved (I thought I should tell this before you get all excited), so it's more of a 'browsing accelerator'.
From Slashdot news: Web accelerators first came around years ago, and they didn't live up to the hype. Now TV commercials are advertising accelerators that speed up your dial-up connection by up to 5 times, they say. AOL and EarthLink throw them in for free; some ISPs charge a monthly fee. Tests by PC World, PC Magazine and CNET show that they do speed up your surfing quite a bit using improved compression and caching.