It works by using compression technology at the webserver end and a piece of software at the PC end. Therefore it only works if you visit a website which has (presumbly) purchased the server-side software from onspeed. It won't affect the speed of other websites or any other online activity, such as online gaming, email, etc.
Therefore once you've paid your money, you're at the mercy of website owners paying for their bit. Sounds like it's destined to fail, to me. But then I said that about the internet 15 years ago /forums/images/graemlins/frown.gif
It does work, I know a couple of people using it, and when Which did a test on several comparable products, this one one of two they said actually did make a significant difference to speed.
It only works on downloads, not uploading from your computer. It doesn't work particularly well on images, particularly those which are already compressed, so there are setting to download images of lesser quality than original, which is fine if you just want a quick look, but you'll still have to download at normal speed if you want an original quality image (I'm generalising here)
The way it works is quite simple. When you download a page from a website, instead of requesting that page directly, the request is made via the Onspeed servers, which are sitting on a very fast internet access, and they download the page for you. They then compress the page using patented algorithms, and send the significantly smaller amount of data to your computer, so it's quicker to download. Software on your computer has the same algorithms, and expands the data.
It makes a significant increase in speed for day to day internet access, as long as you accept that images and already compressed files won't download much faster than normal
Have a look at the reviews section of their website, and read the Which write up.
That said, I know someone who tried it and said it was rubbish, especially when compared to the cheaper broadband products available nowadays. On the other hand, I never much trusted his opinion about much to do with IT /forums/images/graemlins/smile.gif
If it works though, it's helpful for those who cannot get broadband. See here for the faq: