The way that PHP works is that if you send 15 parameters to a function which only requires two, then PHP can more or less "swallow" the other arguments (they're still accessible, but they are less than optional). In your case, I would create a Users controller, perhaps with a display function which only takes one parameter, the user ID. That way, you can have /users/display/1234/whatever/you/would/like/zodiac_sign/stop_sign/favorite_letter/favorite%20punctuation/... Need I go on?
You can get around the need for a controller/method as part of your URL by using the $routes config file... In this case, I don't think user/display would really hurt SEO. I would not use mod_rewrite to do that simply because the architecture is already there in CodeIgniter.
GETparameters? or query stringhttp://www.mysite.com/searchterm/funny/since/2011/sortby/ratings/shorterthan/3/page/5