I have been using Onspeed for the last two years, firstly with a dial up connection and then laterly with a 1meg broadband.
It does work and appears to speed up downloads etc.
There are a few sites that will not work with Onspeed activated. Once you find these, just disable Onspeed for that particular site. On some sites, Onspeed will actually slow down the download speed. That may just have been my computer, not too sure.
I have chosen not to renew at the moment, but I am sure that you will not be dissapointed with the results.