My DD is entering 2nd grade at a school that has been doing Investigations for 4 years, so she had it for all of K and all of 1st. While I don't love it, it's not as awful as I thought it would be. Her school definitely supplements with some "drill & kill" work to cement the basic addition/subtraction facts and I think it can be a better way of teaching kids an understanding of what they're doing (e.g. not just teaching them to borrow/carry in multi-digit addition/subtraction, but teaching them the relationship of 100's, 10's, 1's, etc. and what they're really doing when they do addition/subtraction). In the early years, they do a lot of work with manipulatives and at least at our school, there's not much differentiation before 3rd grade in math (they do some challenge packs in 2nd for those kids who whip through the regular work, but it's pretty much whole class instruction for math for the most part).
We do some supplementing at home (following her lead) with online math programs and Singapore books, but not on a regular basis during the school year. I'm a little worried as she gets older, but we'll probably continue to supplement at home (I have a degree in Math and Biology and my DH has a PhD in Physics, so we're both pretty comfortable with higher level math).