Moving from one helpful gem to another is not too bad. You can even pretty much rewrite an entire application in haml in a single day (we did it between two pe

Moving from one helpful gem to another is not too bad. You can even pretty much rewrite an entire application in haml in a single day (we did it between two people in 6 hours) so it's not something to hold off starting a project over. With that being said, I would heartily recommend haml to anyone using rails (though if you don't know the pain of erb, you'll never fully appreciate haml). Same goes for formtastic - how many forms do you have where it's important enough to delay starting over a helpful but not necessary gem? I think, in general, don't overthink which gems you need to start your project. The answer really is no gems are needed to start your project, and if you just start using gems at the beginning of your project without understanding why they can come in handy, you are finding a problem to fit a solution. As for authentication, I am a big fan of rolling my own auth. Once you have a nice functioning authentication system you worked out yourself, moving it from project to project is simple. The problem I have had with all the authentication plugins, is they insert too much automagic in something that should be simple to understand, and realistically, is a pretty easy concept. There are many guides on authentication, and playing with it is a great way to learn several fundamental rails security ideas that will be helpful in other areas (log filtering, attribute accessors on models).