I initially thought they were a waste of money, but once I saw that 20th Anniversary Edition I was fiendin'. After getting a $50 gift card and $100 cash from the in-law's for Christmas, I jumped on one.
Really, really like it. Even though I have the original SP and a DS, I prefer the screen to the SP and the button layout to the DS (I naturally hit Y and B, not B and A on the DS). Inspired me to dig out some of the GBA games I haven't really spent much time with!