This is a nice curl and wget replacement that handles a bunch of modern use-cases without a lot of hard-to-remember command-line flags.
That said, there is a broader problem of "hard-to-remember command-line flags" which I have personally solved using snippet management (I use notational velocity or command history, whichever is handiest).
There is no doubt httpie's interface is a lot better, but it creates another problem (again, which is somewhat universal) of installing, learning and remembering to use a new tool. This is a non-trivial problem that is a key concern for anyone evaluating a new tool, and it's a problem that only really gets solved with ubiquity.
Finally, an observation that so many of our "traditional" command line tools pay no attention to usability because, at least back in the day, the problem they solved was hard. People had a choice: either put up with an (admittedly) bad interface or write their own version in C. The individual cost of learning a bad interface outweighed the cost of rewriting the tool, and so standard tools were born.
And now, decades later, new generations are stuck having to learn needlessly obtuse interfaces to standard tools. We have a situation where newcomers pay the cost of developer UI laziness in perpetuity. This is, of course, a terrible outcome and it's projects like this one that are trying to change it.
So I applaud the effort and hope it catches on, become ubiquitous, and I can take the curl and wget snippets out of NV.