I've been using Singapore with my oldest for 3 years and just started 1A with my 5 year-old. I think it's a very solid program and love how they teach mental math.
The only thing I didn't like about it was the number of different books there were and the constant flipping back and forth between them. We use all the books except the extra practice, so that's 5 books per child, plus the HIG's. With the new levels we just got, I had the binding cut off all the books, arranged the pages how I wanted them, and then got them spiral bound. So now each child has a workbook and I have the HIG's. Now there's nothing I don't love about SM.
It's not too teacher intensive. I do work with each child almost every day and then they work independently on the workbook pages, word problems and tests. I do all the suggested activities and it's still not too bad.
My girls both love it.