I was a bit of a math-a-phobe. I had never done well in math during my own schooling and it was the subject that caused me the most anxiety when thinking of teaching my own sons. First off I tried finding the one math program with no negative reviews at all, after reading about dozens of different options, I came to the conclusion this program doesn't exist. :) So I tried looking at why programs worked or didn't work for families and chose on the basis of what I think would be the best match for us. Right start taught conceptually not just memorizing algorithms, that was important. It was hands on learning, not worksheet based, and that was important too. What really sold me on the program though was that when I was researching it and reading the materials on it available, I was learning myself. In explaining what and why they teach as they do, lightbulbs started going off for me and I found myself understanding concepts in a way I never had before. Math started to become interesting and fun for me, and if this program could change my attitude that way, I figured it would be a great one to start out my children's formal math learning experience.
So far so good, I have one child using Level A - we enjoy our math time and he is definitely learning. I like that when a concept isn't clear to him, we can "sit on" the lesson for a while by playing the related games - thus practice is not drudgery, or a battle. There are some things to get used to as this is very different than the way I was taught math, so remembering to be diligent about using the abacus and encouraging subsidization (knowing the amount just by looking) rather than counting takes a while to practice. He is excited about math and gaining an ability to understand how and why numbers work. Since the teacher outline is clearly laid out with what the the child is to be learning and doing with each activity, it is also easy to do any tweeking that may need to happen - i.e. skipping over something that is clearly understood or spending more time on something that is not yet grasped.
The only cons I can think of are:
1) The amount of teacher involvement - I am ok with it, but I only have one younger sibling to contend with - with multiple ages this may be more difficult. However, a math professor I really respect (a homeschooling mom also) said that to truly build math skills math teaching does need to be teacher intensive in the early years - later when building on the foundation, it is easier to have a less teacher intensive curriculum. I have heard that the older grades in Right Start are easier for students to self direct. You can also have multiple ages/stages practicing playing the games together and limit the time of the lesson that you need to be leading. So each child would get 20 minutes of instructor interaction and then stop that part of the lesson, even if that means stopping part way through the lesson outline. It is easy to pick up the next day and continue on. I know of many parents who use the program that way.
2) Costs- It is expensive to start with, especially with all the manipulatives. However, you do use the manipulatives throughout the enter program, so the per year cost declines and the per child cost too. It also resells well and if you want the first edition, you can buy it used.
3) Some children may not enjoy using manipulatives and games - this would not be a program that would suit their learning style.
Grade levels used
Teaches the underlying concepts of Math, Is games based and fun, The Teacher Manual outlies what you are teaching and why, No traditional Worksheets