My dc have attended schools with special programs that admit students from many catchment areas - "magnet schools", if you will.
They attended a school with a language immersion program.
Then they attended special gifted programs, starting in mid-elementary school.
For high school, they have chosen to leave the gifted program to attend a performing arts high school. My ds has been there for a year now, studying music. My dd will start in the fall, in the drama program. This school in particular is very well known in our area, and students attend from outside city boundaries - some commute in daily by train.
Overall, the experiences have been positive. I think there are a few reasons. The school population is self-selected to a certain extent for those who value education. These are families who care enough about their children and educational values to examine different schooling options. That alone makes a difference. In addition, the parents and students have agreed that the special focus of the school (language, enrichment, arts, sports, social justice - whatever the flavour of the school) is worthwhile and important. So overall, there is a congruency of values, if you will, between the members of the school community, even though there may be a fair amount of diversity in terms of culture, race, economic level etc.
This social and cultural diversity is another important facet. Many neighbourhood schools exist in a fairly homogenous area. Attending a school outside the neighbourhood is an opportunity to meet and mix with different types of people.
I find schools with a special interest help promote a sense of community and a recognition of the contributions that every member makes to that community. In short, they are happy places where the students are motivated and have a sense of purpose. That reveals itself in improved academic outcomes. Of course, this kind of atmosphere can be created in a "regular" school, and often is. I just find it a fairly common trait in special interest programs.
So what are the disadvantages?
There may be only 1 or 2 classes per grade - meaning your child is with the same group for many years. Social systems become rigid - cliques are impossible to break. Since there is sometimes a sense of "we're special because we go to a special school", it's a good atmosphere for social bullying to flourish - among the students AND the parents.
There can be a sense of competition, although I think competitiveness exists in every school and parent group, not just magnet schools. It may be heightened in a small, focused group though.
Inconvenience. If the school isn't in your neighbourhood, your child will probably have to get up early and get home late from school. There are transportation issues. It's harder to organize extra-curriculars. If the school has a huge catchment area, it's 100% guaranteed that your child's best friend forever will live as far as possible from your home at the other end of the catchment area. Group project members will live equidistant from each other - there will be no one convenient meeting place for all. On the positive side, you will get to explore neighbourhoods that you never knew existed, while you are dropping your child off.
Good luck with your research and making your decision. I think it's wise to explore different options and find a good fit for your child. Take comfort knowing that you can always change schools if it turns out that a school isn't a good fit.