Krosno, the quality upgrade is significant, and that's with the stock speakers. I haven't changed my speakers because right now, I don't really I need to. It sounds that good.
On other thing. You will really appreciate the nice factory look when you see it personally. When I first saw pictures of this HU installed, I thought it looked great but not perfect. But now that I have it installed, it truly does look like the 4Runner could have rolled off the line with this stereo instead.
Try it. If you want to go extra yard, spend the money on some good speakers and the Scion HU. Otherwise, as in my case, the $100 or so you'll spend on the HU alone will go a long, long way in upgrading the sound.