I don't understand some details about Nix when people talk about it. Forgive me for being ignorant.
* Do packages need to be 'ported' to Nix or its build / install system? If so, how much effort will it take and on what factors the effort required to port may vary substantially? (eg: Programming Language).
* How is disk space usage? Is there any kind of deduplication?
* How are security updates to shared libraries handled?
* Transactional upgrades look great. But how is the user experience around Transactional upgrades of running GUI applications?
* Other performance characteristics - bandwidth (delta upgrades possible?), startup time (I hope this will be good)..
> * Do packages need to be 'ported' to Nix or its build / install system? If so, how much effort will it take and on what factors the effort required to port may vary substantially? (eg: Programming Language).
Yes. To make Nix really work it has to be pervasive and this sometimes means porting packages again, however there's a lot of automation around this, see[0] for ones that leverage existing build systems in other languages. Autoconf and cmake packages are already accounted for by the default builder, and usually only the dependencies have to be specified.
> * How is disk space usage? Is there any kind of deduplication?
The Nix store can be intensive on memory, as once a dependency such as glibc is updated, all the reverse dependencies (i.e. the set X such that X depends on glibc directly or transitively) have to be rebuilt as well. There is an option to optimise the store, but I don't know how much it helps in practice, see[1]. Garbage collection can be invoked manually or automatically to free up unused store entries.
> * Other performance characteristics - bandwidth (delta upgrades possible?), startup time (I hope this will be good)..
By startup time you might be referring to NixOS. It's very good on my Late 2013 13-inch MacBook Pro, around 20 seconds to go from cold boot to login screen.
[0] https://github.com/svanderburg/node2nix https://github.com/NixOS/cabal2nix https://github.com/cargo2nix/cargo2nix https://github.com/nix-community/poetry2nix https://github.com/kamilchm/go2nix https://github.com/nix-community/pip2nix