What third-party services do you use for panel testing? what are the pros and cons of each? How you used these insights to impact the product roadmap with PMs?

I've used a few different panel providers for large-scale segmentation studies, messaging feedback, and more. I won't call them out by name here, but I'll suggest things to look for as you evaluate options: 

1. How well can they recruit against your unique market, and how will they make sure to meet their targets? What will it cost to meet your goals? 
2. When responses come back, how do they filter out bad responses, and what visilibity will you have to the data coming in (before you're out of field). 
3. What support do they offer for survey design to make sure you get the most value from their breadth of experience? 

In terms of insights guiding the product roadmap for PMs, I've found segmentation studies to be an incredibly powerful tool to align the organization on strategy. It sizes the market, identifies what personas to prioritize, provides rationale for what pain points to solve for first, and creates a lock-step experience from product development to messaging and launch planning to sales enablement. 

For smaller-scale, more acute decisions, working with PMs to identify specific questions they need to answer at a regular cadence (say, top questions for a given program increment) to drive product strategy is also a great way to bring value to the table. 

We have used usertesting.com and Survey Monkey panels. Usertesting is better for qualitative insights and SM is better for quantitative and quick results. It's impt to be aware of panel quality and to use the feedback accordingly. For example, you might want to validate what you hear from a panel by using multiple sources. PMs have been involved in development of questionnaires, we make it easy for them to see results and they use the results to inform roadmap priorities.