I've been using Investigations for the past few years. Investigations had been implemented into my school district a few years before I started, but fortunately I have a great college methods course and student teaching experience. Since I've been teaching, I've attended several Investigations workshops. Many of the more experienced teachers in my school were very frustrated when the district adopted the curriculum. It is definitely a new and different way of teaching math compared to how it was being taught.
Our students (we go up to 5th grade) have had Investigations since Kindergarten. We see such a difference in how they think about math. I really like Investigations. I like the way it makes students think more abstractly, has real like problems, and encourages students to use their own strategies for solving problems.
There are times I feel that Investigations spends a bit too much or too little time on certain activities/concepts. If I feel it spends too much time, I will pick up the pace and condense the information/activities. If I feel my students need more time, I will supplement what I feel is needed. We also use Calendar Math and Practice Counts for curriculum, so I feel Practice Counts gives them the extra practice that Investigations may lack at times.
I agree with Amy...number sense...my students (even those who struggle more) have great number sense. Some of them can solve problems faster than I can. They've developed the number sense that I've been developing since I began teaching Investigations. Unfortunately some teachers still use the "old" methods (for example-borrowing and carrying for subtraction). When the kiddos come to me using that method, I don't tell them they can't, but they (and I) quickly figure out that it doesn't make sense to them and is more confusing.
Our parents were also apprehensive, although it seems to be getting better. It was difficult for them, because they are used to the "old" methods and would help their children using the methods they knew (which seemed to confuse the kids more!). I've definitely seen that parents are understanding and trying to help their children in a different way.