I think we all struggle with this in the CPG industry. There are some great firms out there thought.
Kaleidoscope: Kascope.com- I have done some work with these guys and they have a strong strength in understanding the consumer. The also do quite a bit of work for P&G.
Product Ventures: I have not work with them directly, but they have done some work for us in the past and everyone here in the office seems to think they do a pretty decent job. I know they have a decent understanding of pkg engineering as they have pkg engineers on sight as well as mechanical. This you do not find often.
Group4: I know a few people that came from there as well as they have pitched their agency to us. I have never worked with them, but from what I have seen they have an understanding of consumers interaction with packaging and how consumer shop. Their design work looked decent, but like I mentioned I would have to give them some work to get a true read on their skills
There are others such as:
Nottingham and Spirk
All of which I have worked with and do really great work.
Hope this helps.