I can speak with direct knowledge on this one.
If you are speaking of college prep classes then you are speaking of my daughter's situation. She took the college prep classes in high school and not only did she learn more, she went on to get many scholarships for college.
For any student who can handle the stress (and yes, these classes can be highly stressful, with much more homework than other classes) I highly recommend them.
And to show the apple rarely falls far from the tree, I took enrichment classes all through my years (these are not college prep courses, but rather, classes that allow students to move beyond the regular curriculum at an accerated pace), and I will say that I honestly learned a lot, and through it all taught myself how to educate myself, which has become a handy skill over the years. :)