I used Next.js to build http://user-agent.io/ over a weekend and I'm really happy with the results so far. Dynamic url routing is a bit awkward, but not onerously so. (It also uses Express to handle WebSockets and dynamic routing for the share/host pages.)
Source is at https://github.com/nfriedly/user-agent.io if anyone is interested.
I'm thinking about switching some of my work projects over to it sometime soon.