Two of my boys, ages 6 & 7, started Horizons 1 for first grade last year. We are almost finished. 6yo is 'mathy' and does great. 7yo not so great. Horizons did not provide enough explanation or concept development for him.
I just switched to Singapore. I bought first grade (1AB) for 7yo. We're flying through it trying to discover his gaps in learning and fill them in. I will start 2AB with both boys in the fall.
When I received Singapore, I actually cried tears. SO much explanation in various ways, colorful textbooks, fun ways to approach problems. My 7yo asked me, after four pages of math, when we were starting "real" math because he was having too much fun!
I bought the CA Standards edition for earlybird kindergarten for my 3yo (he, too, is 'mathy') and 1AB for 7yo and 2AB for both older boys. You can look at sample pages on singaporemath dot com for more info. We couldn't be happier!
I think Horizons is great for 'mathy' kids who 'get it' without much help. I think that's a gift from God. But for those who struggle, I would never recommend it. I was going to go with MUS for 7yo but I just realized that that would put him so far behind his 6yo brother in math skills that he might not catch up for some time.
IMO. : )