How far along is the company? What's your current status, do you need the money? How much do you believe in the product? What % of the workload is going to fall on you, and how much do they need your skillset? How do their resources look (money, time, etc)?
There's a lot of things you'll need to explain before being able to get a good response. i.e. If the product is from scratch and there are little resources, there's no reason you shouldn't get close to or even founders equity, but if they already have a product with traction it's a completely different story.
we've been using mixpanel for our facebook apps since launch and it's helped us greatly, and our entire team loves it. even the pm understands the stats without asking questions (which is usually not the case).
understanding how a user uses the application has not only helped us figure out what features the user wants, but also which of those actually WORK (in real time).
There's a lot of things you'll need to explain before being able to get a good response. i.e. If the product is from scratch and there are little resources, there's no reason you shouldn't get close to or even founders equity, but if they already have a product with traction it's a completely different story.