While I personally prefer 3.5Ed, most of the people in my current game group (esp. DMs) haven't made the move, so what I usually get to play is 3.0.
IMHO, the only solid advantage 3.0 has over 3.5 is found in spell durations. Several (mostly buff) spells had their durations axed from hours to minutes. That is a bit of a pain, but overall, it's probably better for game balance. Were I the designer, though, I would have bumped the spells' levels instead of shortening their durations.
In addition, there are several good products that simply never got updated for 3.5- at least, not as a full product release. Oriental Adventures was updated only in Dragon #318 (as I recall), and Green Ronin's Book of the Righteous is still only available in 3.0 form AFAIK. The same could be said of several fine products from AEG and other 3rd party publishers...some no longer in business. Of course, that means you can probably assemble a nice 3.0 library via the used book market for not a lot of $$$$. Besides the ones mentioned above, I'd add FFE's Book of All Spells and AEG's Feats and Mercenaries to the list of quality 3.0 products, as well as some of the games like Arcana Unearthed or Swashbuckling Adventures.
Still, 3.0 remains a solid game. I just wouldn't play it if I had the option of being in a 3.5 setting.