There's a good piece about onspeed in the current PC Pro.  Its conclusions are that the compression methods it uses are good for pages that don't already have compressed stuff on them, such as jpgs, but not much cop for image-rich pages, which of course most are these days. 

Worth a read, though - but having read that I wouldn't particularly recommend onspeed to anyone on dialup. 

And by and large I think most dialup acceleration software isn't great, either.