I agree with your sentiment in general, but what do you suggest to use in place of devise? Surely you're not suggesting to write all the code for handling mail confirmation, password changes etc. yourself?
Furthermore you usually end up heavily customizing whatever auth-code you start with anyway. The shrinkwrapped gems never cover even half of what an app eventually needs. Thus another advantage is that you won't have to reverse engineer devise or authlogic when you arrive at that point (and trust me, you really don't want to look at their code and the SQL they emit).
That all said, in a recent project I've found the 'sorcery'-gem to provide a reasonable baseline for rolling your own. It's one of the first "second generation" Rails-gems that refrain from most of the magic and instead provide a set of primitives that you wire up yourself.
It still suffers from a bit of rails-smell (code-generators...), but overall the level of abstraction looks about right.
Task that with another product I just launched: over 100K users, on Devise, no customization needed, and Login with Facebook took 1 additional library, 3 lines of code and 15 minutes.
I used to be the roll-your-own-everything guy, years ago. After maintaining dozens and dozens of Rails apps, I'm not anymore. I'm much happier when logic is pushed into libraries and I can ignore it until it matters.
> In fact for serious apps that are expected to grow I do recommend to write the auth yourself.
No, no, no, just no. If you don't grow, you just wasted time you should have spent shipping real features. If you do grow, refactor in your own authentication later if you have to.
> you'll fully understand what your code does and when
You will, for sure. When you leave for another project and I'm brought into maintain $app, I won't, straight away.
> there will be no guesswork in one of the most important areas of your app.
There is no guesswork with 3rd-party libraries. `bundle open gem-name` is your friend. If you read through the code, you'll understand what it's doing, just like I'll have to read through your code if you roll-your-own library.
One of the major benefits to standardized libraries is that I only have to read the library for Devise or AuthLogic once across a dozen apps. I have to learn each customized solution 100% of the time.
> It still suffers from a bit of rails-smell (code-generators...)
I get the feeling that you work on really large apps with a lot of custom logic, and end up wishing you had complete control over everything. In such situations I'd probably end up agreeing with you 90% of the time if you suggested throwing out a 3rd-party library and rolling your own.
I just strongly disagree that you should start out that way. Your app should have a single selling point: "Schedule my tweets", "Remind me to pay bills", "keep track of my tasks", or whatever. Not "Log in with our unique authentication code."
No app ever became a million dollar company because a developer thought the most important job was to roll their own authentication scheme before they even scored a thousand users.
You are of course right that rolling your own auth won't be the first priority in your initial PoC. Neither is it needed for very simple apps or when you're dead-certain that you won't need more flexibility than the common gems provide.
However in my experience the latter almost never applies in a commercial app. Suddenly you need OmniAuth in addition to devise, and some form of ACLs. Then you grow an API that also needs some sort of auth-tokens. Then there's this other site you want to interface with which needs yet another bridge. Then one day you run that ad on TV and learn the hard way that those extra-lookups devise makes on every request are not free after all...
So what I'm saying is that the design of (in particular) devise and authlogic is just not a very good one to start from if you can already predict that you'll need customizations (beyond templating) in the future.
A frankensteined devise can be a lot harder to understand than a straightforward impl from scratch - but in the end it of course also boils down to who wrote it and whether he wrote it for the first time.