We have had Netbooster deployed on my PC and the laptops we have given to pretty much everyone in my company with Reliance Netconnect service - about 6 of us. In addition, we have 15 other employees who have been using it with their cell phones in phone-as-modem modem. We have all had a very good experience with it. In fact, we are making sure that all our laptops and PC have the software installed goinf forward.
I had heard about the provider of this solution before, a company called Venturi Wireless, while I was in the US a couple of years ago. One of the really large CDMA providers there, Verizon Wireless, were also offering their software. That was for a 3G network, and I was sure that the solution did more than compression.
I did some reserach on their website http://www.venturiwireless.com, and there is a lot of interesting information. They are doing a lot more than scaling down images through compression as mentioned by you. It looks like they have solved the transport layer problem that we see on wireless links.
Personally, I have been using netbooster for a few months and I cannot live without it. I have had situations where I could not even open a webpage using Netconnect, and the only way to open the webpage was by using Netbooster.
Now I have just found out that Tata Indicom is offering a similar solution called Speed+. One of my employees who has a Tata data card had informed me, as he was feeling left out without a Netbooster type software. I'm going to find out more about that and post.