Freelance taking shares into startup: what's your opinion?

Hi folks,

I've been working as a freelancer since I graduated, it's been 5 years now. I specialize in developing web-based softwares for early stage startups.

Being a freelance has tons of perks, but sometimes when I meet an awesome startup, it feels like I am looking at the window and missing what's inside.

I don't want to leave freelancing and become an employee, at all. However, I'd like to take a more active part in my client success.

I think the way to conciliate both could rely on adapting contracts and pricing so they are more success-based instead of being 100% time-based. I am ok with taking risks, lowering my hourly rates and maybe never see the success fee.

Do some of you have experience with this approach? For instance taking share in startups you work with as a contractor, or maybe billing "success fees" based on some metrics or when the startup "exits" in exchange of a lower hourly rate?