Yes, many existing CD players have auxiliary inputs, and many people plug portable MP3 players into those inputs. That will work just fine. You can also sometimes adapt a car's factory sound system to accept aux-input if you find a company that sells the right adapter.
But if you're looking for the best feature set, in a device that was specifically designed for in-dash use, then the Empeg is still the best product of its kind.
Before you decide, hang around here a bit, have a quick skim through the
FAQ, and see if the empeg is the kind of thing you want.
Those of us who like the empeg are a very specific breed. We consider maximum audio quality and maximum song storage capacity to be the ultimate goals. Secondary benefits to the empeg involve things like the visuals, the hackable Linux internals, and the player's networkability. These are things you don't get with a portable MP3 player.
If those things appeal to you, then definitely consider an empeg seriously. But if all you want is to play a few MP3s in your car, then the plug-in-a-portable scheme is fine for you.