Excellent, just did the same thing about a month ago (for https://codeable.io). We wanted to use Rails 4 but Active Admin didn't want to play nicely, so I looked around (and checked it's dependencies) and came to the same conclusion. One additional gem I'd recommend with this setup is also Ransack (AA also uses it) that allows you to easily implement searching.
After I was done, I wrote pretty much the same amount of code I used to configure AA.
I love SimpleForm, but I still wouldn't want to roll my own administrative interface. With RailsAdmin, I make changes to my database schema, or add a new model, and those changes are automatically reflected in my admin interface. One less thing to maintain.
Also, I don't have to write any CSS, I get PJAX thrown in for free, and I inevitably have clients say "oooh, pretty" without any extra effort on my part.
They also say "How did you build this so quickly?", which I have no complaints over.
After I was done, I wrote pretty much the same amount of code I used to configure AA.