Bought this new, and I'm STILL playing it regularly! Alongside UT3, in fact often both on the same night. UT3 environments have, as one would expect, much more detail and complexity. But playing them side by side, many times I find myself wondering if they're really an improvement. 2004 just got that many things right. One of the all time PC classics.
There is a Creative X-Fi-only sound patch out there. The sounds are better, no question, but they are also just plain different. Having lived with them, I won't go back, so yes I do recommend them, but a word of warning that they are definitely different, not simply better.
Unfortunately 2K4 is a backdated title. At $9.99, it's a bit steep. When you can pick up UT3 for as little as $15, you should consider the playtime of your investment. UT2004 servers are at the tail end of a great career, UT3 is just hitting it's stride. It is and always will be a great single player game, but for years of play value you need to go with the title people will still be playing in a few years.