I think this proxy stuff is over-rated and the concept of privacy is distorted. There are plenty of ways to toss your privacy out the window on the Internet and your IP address is at the very bottom of that list. And if you really need to hide your IP then (a) I can't help but wonder about the legality or propriety of what you're up to, (b) your first inquiry should be whether your anonymizing service is subpoena-proof. Many are not. The Tor network is interesting in this respect, since if nothing else it substantially complicates inquiry efforts.
Let's separate fact from fiction and potential from practice...
Is your IP anonymous? Mostly, unless you have your own assigned address space (I do) or you're doing your surfing from a business or enterprise that has its own assigned address space. Finding out who is behind an ISP's dynamically assigned IP takes a letter from an attorney. Hardly an automated process.
Can your IP address reveal your geographic location? Yes, but only roughly. It will not reveal your address. At best it will reveal your town and in some cases, even that may be somewhat inaccurate depending on how your service is provisioned.
On the other hand if you fake your IP, any web site that uses geolocation services to target advertising, will offer you ads that are irrelevant. I'm on the fence with this. Which is worse? Relevant ads or irrelevant ads? Those of us who are experienced pretty much ignore all ads anyway.
Can your IP address be used to track you? Absolutely not. I can walk out the door right now and although I'll have to stop typing, I can guarantee you that nothing about my IP address will have given away the fact that I went to get a bottle of Coca-Cola from across the street.
Can your IP address be used to track your [web] activities? Yes, to a limited extent, but due to the prevalence of dynamic IP addressing and the number of proxy -based systems (frequently used more for security in large companies than for privacy), generally nobody bothers. Cookies are a more reliable way to associate a person (or at least a computer) with their browsing habits. But for that to happen successfully some cooperation is required among the sites that place those cookies on your computer. Don't even get me started about cookies - another overblown source of panic.
SSL proxy connections. All very well and good for keeping prying eyes out of your local data stream but useless once the data leaves the proxy and finds its way to the web server. So at least it'll keep your ISP and your neighbor's young hacker kid off your bits 'n bytes.
Some of the other options really slay me. Why block info about your OS and your browser version? Seriously! And why strip HTTP Referer header info? A lot of this is used to help improve the web browsing experience for a viewer and in the case of referers, helps protect the site from abuse.
There are some other interesting purposes for web proxies that aren't directly related to privacy, such as circumventing ridiculous limitations on certain web services. It might help with the quality of your search results during your stay in China, or help facilitate access to P2P applications that are blocked from USA IP addresses, or help travelers gain access to USA web services that block or offer reduced functionality to offshore IP addresses.
A final note is that web browsing these days often involves a lot of extra plug-ins, multimedia widgets, peer to peer gadgets and so on. Not all of these blindly play along with proxy settings or proxy stacks. Some will do their own thing and some simply won't work at all. You'll have no idea if one of those pieces are dancing to their own beat until you run packet capturing software - a job not well-suited to those without propellor beanies. Those of us with the beanies know better than to care.