Hi, I just graduated from an Early College Program. There are a few benefits and a few drawbacks, but that can be said with anything.
For one, I learned how to adjust to a higher rigor of academics compared to some of my friends who went to "normal" high schools. Going to class at 7 A.M. to sometimes 9 P.M. (this schedule varies between Early College high schools; some do not allow evening classes). It gets tiring, but you learn to adapt. I somewhat knew what the college academic aura was like because of this exposure.
Another positive trait is that you get a lot of GEs out of the way. This saves you money, especially if your sister or whomever decides to go to a public university; they usually take all of your units. This will also save you two years (depending on your major and what you take) of college tuition.
In terms of drawbacks, you have an easy way to mess up your college GPA. Your college GPA follows you wherever you go. At an early age, I don't think you realize how detrimental this can be to your future. I believe that some people are forced into these early programs, do poorly and then realize how badly they've messed up their GPA later on in life. It sucks, frankly.
All in all, I loved the program I was in. I met amazing, inspired people who've followed me from middle school and I believe that being in that program has strengthened and put us ahead of a lot of others who are just starting out in college.