We recently switched from Right Start B to Mammoth. It was a good decision. DD needs to spend more time on concepts, then move on to something new, then go back, kwim? She excels in reading, spelling, writing, but math is not her strongest at this point. I kept the manipulatives that came with RS and am using them with Mammoth.
DD enjoys the worksheets and thinks it's fun. She really does enjoy Mammoth more and I do too as well. It's not as teach intensive, which is nice when you have little ones running around.:D
I am thinking about moving on to Singapore for 2nd grade, but I am still researching.