I've been using Favor in Austin for probably a year now. It's useful. Adds up though. Example from Sunday: $8.12 for some amazing pho, $5.87 auto-suggested tip for the runner, $.41 processing, $5 delivery fee, and the cost of my dinner more than doubled to $19.40.
I've been using Favor since they launched in Austin. Fantastic service! But like Shlok mentioned, it can add up quickly depending on what you order. I've found it works better for larger groups of people (like a hackathon or lunch meeting) as opposed to a solo dinner. It's also much more convenient than the newly launched Uber Eats. They deliver at all hours of the day and even into the wee hours of the morning, for those of that mysteriously start craving tacos at 2AM.