Hi! Devenv user here:
devenv.sh autogenerates Nix specifications of container images for each of the development environments you define with it, and exposes them in a Nix flake for you. But if you don't explicitly ask Nix to build those for you, it doesn't.
My team's most complex devenv environment passes through some packages from Nixpkgs, exposing them in the flake, defines 3 or 4 development environments ('devShells') and also exposed (and includes in some of those devShells) some custom packages that just live in that repo and not in Nixpkgs.
Our simplest just uses all built-ins and defaults, exposes no packages, doesn't have a custom flake... there's basically nothing to it. It just uses devenv/Nixpkgs built-ins to add OpenTofu with some Terraform plugins to the path and plugs a Terraform linter into the project's pre-commit hooks.
There is some support for running long-lived processes via process-compose, which is inspired by docker-compose, but we don't yet need it for anything so we don't use it. (In the past I've rolled my own solution for this with Nix, direnv, and supervisord. Today I'd probably just use devenv.)
What I love about devenv is how cleanly it integrates with the wider Nix ecosystem in a no-nonsense way, as well as its great docs. I like that it
- it uses the Nix module system for configuration, based on a standard implementation (flake-parts) shared by other projects
- what it builds in, i.e., pre-commit hooks integration, exists in a separate repo also usable on its own via flake-parts
- the docs and the tool both support flakes-based setups, non-flakes setups
- if you want to let devenv automagically set up a flake for you, you can, but you can also use most of its features with a handwritten flake.nix customized to your liking
- while it exposes some configuration via yaml for simple uses, the full power of Nix is always available
- it takes care of all the little details you'd have to deal with if rolling your own (e.g., garbage collection, direnv integration) in a sensible way
For me, as a Nix user, it does a good job of keeping simple use cases simple, it has low lock-in, it uses familiar configuration interfaces, and it doesn't have too much magic.
I hope that helps you get a sense of what using devenv is like for simple use cases as well as what the more complex stuff might look like!