Sure. Free 2-5x speedup. pypy + pypy's pip generally works as a transparent drop-in replacement to python + python's pip, so it's free speed.
It doesn't (or didn't) work when you need to rely on an extension that uses Python's C API. I haven't followed the scene in awhile so maybe that's changed. pypy's pip has so many libraries that I hardly notice, so maybe they solved that.
Unfortunately python is fundamentally slower than lua or JS, possibly due to the object model. Python traps all method calls, but even integer addition, comparisons, and so on are treated as metamethods. That's the case for Lua too, but e.g. it's absurdly easy to make a Python object have a custom length, whereas Lua didn't have a __len__ metamethod until after 5.1. I'm not sure it even works on LuaJIT either. Probably in the newer versions.
I can't tell what you mean by the last paragraph there, but oftentimes PyPy's speedups come exactly from inlining stuff like what you refer to there -- Python's not fundamentally slower, it's those kinds of stuff that you can speed up.
(And yeah the CPython API is still a pain point if you've got a library that uses it, although some stuff will still work using PyPy's emulation layer. It'd be great if people stopped using it though.)
For example, Python makes it fairly easy to trap a call to a missing method, both via __getattr__ and __missing__. In JS the only way you can do that is via Proxy objects, and even those have limits.
You can't always inline the arithmetic ops effectively. You can recompile the method each time it's called with different types, but that's why the warmup time is an issue. This wouldn't be a problem if Python didn't make it so trivial to overload arithmetic. JS doesn't.
Twist: Lua makes it trivial to overload arithmetic using metatables, but LuaJIT seems to have solved that. If there is any warmup time, it's hard to tell. Mike Pall is a JIT god, and I wish we had more insight into everything that went into producing one of the best JIT's of all time.
I'd love a comment/post that highlights the differences between JS and Lua as the reason why LuaJIT was able be so effective. There must be differences that make Lua possible to speed up so much. There are easy ones to think of, but the details matter a lot.
It doesn't (or didn't) work when you need to rely on an extension that uses Python's C API. I haven't followed the scene in awhile so maybe that's changed. pypy's pip has so many libraries that I hardly notice, so maybe they solved that.
Unfortunately python is fundamentally slower than lua or JS, possibly due to the object model. Python traps all method calls, but even integer addition, comparisons, and so on are treated as metamethods. That's the case for Lua too, but e.g. it's absurdly easy to make a Python object have a custom length, whereas Lua didn't have a __len__ metamethod until after 5.1. I'm not sure it even works on LuaJIT either. Probably in the newer versions.