I think SM is a great place to start, especially if you're in a panic (as I was last year, since we pulled DS7 (then 6) out of 1st grade in late Sept. for "emergency homeschooling." No time to plan or even think! Scary!). It is a solid, useful, well-done curriculum.
We did move away from it this year, though we may move back once DS has done some other things. SM--like pretty much every other math curriculum for elementary-aged kids that I've seen--was just too focused on calculation and not conceptual enough for where DS7 is right now. But it's definitely an excellent curriculum, especially for GT kids.
Get the "Intensive Practice" workbook if you like more problem-solving stuff. Some of those problems are downright hard--my DH is an engineer and I'd like to think that I'm no slouch, but some of the problems in the lowly 2A book made even the two of us have to stop and think! The calculations at that level aren't hard, but the critical thinking is really quite challenging. I highly recommend it, and you can use it instead of the regular workbook if you want to. (You just miss all the easy problems that way.)
Also, I like the teacher's guide, but I thought the student textbook was kind of useless. Unless you want to hand your child the books to do completely alone--in which case you want both the textbook and the regular workbook--I think all you really need is the teacher's guide and the "IP" workbook.
(If you want more word problems, the "Challenging Word Problems" workbook is good. I just liked the "IP" book because it contains a wide variety of problem types, not just word problems.)
