For building a HTTP API why would you tie yourself to a closed source non-free framework when there are so many good alternatives out there? besides dealing with the servicestack licensing you now have to deal with all the windows licensing too.
I mean, I doubt it has some secret sauce that makes it better over everything else for building a REST API.
Licensing means there are dedicate developer resources working on regular improvements all the time. Disclaimer, I am one of those resources :).
Regarding 'secret sauce', it's not a secret at all, as others have said it has a big focus on simple message based approach. Recent SO question was asked sums it up.
Whilst ServiceStack is a commercially supported product, it's not closed source. It's available under a dual commercial and AGPL/FOSS Exception (i.e. Free for OSS) licensing model and all its source code is available on GitHub. Whilst Windows is our Customers most popular platform, ServiceStack has been running cross-platform on Linux/Mono for over 5 years and we have several customers that host on Mono, e.g: http://blog.geni.us/2016/02/25/the-geniuslink-technology-sta...
It requires paid full-time developers to further develop and support all of ServiceStack's 60 NuGet packages and multiple IDE tooling support. Unfortunately we don't benefit from Windows Server Licenses or Azure Hosting that's paying for development of all past and future MS frameworks which are clearly benefiting from a superior business model. A commercial license is the only way we can sustain full-time development on ServiceStack, without it it'd just end up as another Framework destined for the abandoned .NET Graveyard heap as have nearly every other .NET Web Framework before it. I believe Nancy is the only non-MS alternative Web Framework that's still actively developed with decent market share.
As far as licensing models go, we aimed for the least friction option with a perpetual, royalty-free per-developer licensing model where any licensed developer can deploy anything they create with ServiceStack to unlimited Servers at no additional cost. Also all our client libraries are also free to use to consume ServiceStack's HTTP, MQ or SOAP Services giving ServiceStack Services maximum utility possible.
Obviously the bar is much higher for a commercial product and we need to deliver more value and productivity to justify the cost of the License. We're fortunate that enough Customers see value in ServiceStack that lets us continue making the most productive Framework and surrounding library ecosystem we can, we're even more fortunate that we're able to be 100% focused on development, we've never paid for any marketing, never paid evangelists, never paid for referrals, never called a Customer, never attended a Tech Conference or given a talk since transitioning to a commercially supported product ~3 years ago, etc. Yet we've been bootstrapped and profitable since Day 1 - all the success we've had has been through word of mouth from Happy Customers. Whilst we realize this isn't optimal for a Commercial Company and may change in future after we've completed the development tasks we've set out to do - it means for several years we've solely been product focused and have poured all our efforts and energy on further developing ServiceStack to make it the best development platform it can be - of which we've seen nothing else like it with a lot of its key features are a natural extension possible due to its message-based Services architecture that we believe already offers compelling value and advantage over MS frameworks that you might be used to.
In short, we believe ServiceStack offers productivity and value over and above its licensing cost and is the reason why our Customers see value in it.
I mean, I doubt it has some secret sauce that makes it better over everything else for building a REST API.