I've only played one night (but 24 fights), but I'm ready to give you the lowdown on "Fight Night 2004," which I'm ready to call the best modern boxing game I've played (I say "modern" because nothing will ever top "Mike Tyson's Punchout" for NES, if only for sentimental reasons).
First off, major props to EA for licensing P. Diddy's "Victory" as the primary song for "Fight Night." For those that don't know, "Victory" sample the fight music from "Rocky," which makes it the perfect video game boxing song.
As for the game itself, the biggest change is in the control system. The days of alternating between the hook and jab buttons as quickly as possible to score a quick knockout are gone. The punches all are thrown using the right analog stick, making for a more realistic boxing experience. It's easy to throw jabs and straights, a little harder to throw hooks, and the uppercuts are very difficult and leave you open. Plus, because of the new system, blocking is more important. You can no longer just "block" by throwing lots of punches.
The control system definitely takes some getting used to. Unlike what I did, you'll probably want to fight some exhibitions first before starting a career. I lost at least three fights to start my career just because I was unskilled at throwing punches.
As for that career mode, it's much more in depth than ever before. You start off ranked 50th (as opposed to 20th in previous EA boxing games) and it's realistically hard to move up. Also, adding to the realism, is you can fight at different venues at early points in your career -- in previous versions, the venues changed with your experience level -- and different venues offer different purse sizes.
One disappointment with career mode is that there just isn't much to spend your money on. You can't buy better trainers or cutmen (at least not to the point I've played) nor can you improve your training facilities (which is the only quibble I have with the fantastic training modules). Also, EA did cut the early hyped feature of being able to move up and down weight classes at different stages of your career. At this point, I almost hope it doesn't show up in future versions of "Fight Night," because I'd hate to have to pay for this every year like I already do with "Madden" and the NBA and NCAA Hoops games.
As for my recommendation, here it is. If you're a boxing fan, or a sports gamer, pick this up. If not, an older version of "KO Kings," with its button-mashing control system, is probably better as a rental for you. But this game is a huge advance for fans of the "KO Kings" series.