How effective are download accelerators?

[Tyler]

I constantly see these commercials between Netzero, PeoplePC, and AOL (aka netscape).  They all speak of these download accelerators that can make surfing the net "up to 5x faster".  Being on broadband myself, I have never used these utilities, but I wonder how effective they actually are.

Has anyone ever used (or maybe currently using right now) these download accelerators?  What do they do and how well do they really work?