I never got a DSi so I only have the DS Lite to compare them too but I find the built-in speakers on the 3DS to be far superior to my old DS ones. I am quite pleasantly surprised by the richness and amount of bass that comes out of them. I used to prefer playing games with headphones when I was home but now I like the speakers better.
 
But maybe that's just me! What does everyone else think? Are the DSi speakers as good?

I cant really compare it to a DS since i never owned one, but comparing it to the PSP's speakers? 3DS are better 
 
the PSP speakers were horrific, i always used headphones on those.

Totally agreed. I also never owned a DSi/XL, but the speakers are so much better than the ones on the Lite or Phat. Oddly they don't make the panel vibrate, but the screen area does quite a bit(front and back). Not a big deal, but it's kinda distracting if you hold it in an unconventional way.