It was a while ago so forgive my dodgy memory
but I recall that in terms of kb/sec, where I could normally pull 300kilobytes/sec from a UK Server (on 3mbit broadband), with onspeed enabled I was able to achieve download speeds up around 550kb/sec. This is well above the theoretical limit of a 3MB connection, 3MB = 3072 kilobits = 384 kb/sec and works out as an "onspeed" speed about 43% larger than my original speed.
The speed increase should appear more significant for 56k users and where download speeds were 7-10kb/sec before you should see around 30-50 kb/sec. This is a massive improvement, and could be compared to turning your 56k modem into roughly 280k "middleband".
The official word on onspeed is that it is up to 5x faster for dial up, and based on the reviews I've seen you should little have reason to doubt them
