It sounds better
Honest. I've done the comparison myself. An MP3 in its highest quality settings, with the best encoder there is (LAME), still doesn't sound exactly like the original. An Ogg Vorbis file encoded using an average of 130 kbps (quality setting 4.9) is indistinguishable from the original.
I did the tests using an expensive sound card and a very expensive set of Technics headphones. The difference is dramatic.
Yes, it's free. Free as in free beer. But, more importantly, free as in freedom. You're free to do whatever you want with Ogg. You do not need to pay royalties for software patents. You're not at the mercy of giant corporations
How do I switch?
You don't need to switch your music collection to Ogg. The best course of action is to just rip any new CDs to Ogg directly. The Play Ogg page has more info about it.
If your portable music player doesn't support it, there are countless player managers that will transcode the files into MP3 before saving them to your portable music player. For example: I have a Palm T|X that doesn't support Ogg (it does, but let's pretend it doesn't). Amarok transcodes each Ogg music file into MP3 before saving it to my Palm's memory card. I don't have to do anything -- it's fully automatic!