Perhaps I’m cynical, but I don’t see it being in the least effective on a political level. I just don’t think that more than the very occasional, low-level politician gives a flying defecation about pro-lifers anymore. I do, however, see it being very effective on a personal/spiritual level, especially for young people.
I went as a teenager, and was blown away. Admittedly, I was very sheltered, but I didn’t really know that there were many young people involved in their faith or in pro-life ministry. To spend a couple of days praying, attending Mass, and then attending the rally and March with them was decidedly lifechanging: it gave me hope, it meant I didn’t feel alone, and it made me feel a part of something much bigger than myself. Really, it was the first time I felt like a member of the church as a whole.
ETA: Also, I can’t help but think that thousands of people attending Mass and praying the Rosary together to end abortion must result in some sort of spiritual good, both for them and for those involved in abortion. At the very least, I can’t see it doing any harm!