I don't use any internet speedup utility. I do not believe the claims -- the FCC in the US regulates the maximum transfer speed over dialup, so that means vendors have to come up with means of compressing data in order to achieve the claims. But then we have to think... what if I am already moving compressed data. It is certainly possible that re-compressing data will make it even larger.
That and I have to wonder about cross-platform compatability. A lot of those packages are saying for Windows only. Hmm. Compressing, windows only, decompressing, slowdowns...
These tools are all scams. The limiting factor on the time it takes to get data to your computer is hardly ever the bandwidth YOU have available (especially if you're on broadband) but rather the speed at which the host sends its data (which these tools can't affect).
And if you're on dialup (unless you're in the stone age and using a 33k6 or slower connection) your data is already compressed as the V56 protocol includes downstream compression (the actual line itself is still at 33k6, but the dialup server at your ISP will compress the data before sending it to you).
And if you install it on your own system, it does nothing at all to reduce bandwidth usage of course.