I havent tried Proxycon but I have tried Propel
http://www.propel.com/
Here is how it works. Pages get sent to their servers and images are compressed depending on your settings. Then you download them from their servers. Thats the reason behind the monthly fee.
It DOES work. They have an official test and I did an unofficial and the results were great on both. Without it, it took 78 seconds to load a page, the first time visiting the page with propel it took 28, and the next time took 23 seconds.
If you have compression really high and want to see a picture in full quality you can just rightclick and show full. You should at least try the free trial. Its 30 hours free but thats only 1 day because the timer doesnt stop if you get offline. So only use the free trial if you got alot of time to play with it.
I look like a walking advertisement :P
edit: Oh yah, This won't speed up your downloading, only browsing.