if your variable is a user then call it $user not $obj - it makes your code easier to understand
Yes, you can mass assign an array to a model, but pay attention to name your form fields exactly the same as the columns on your database. If you don't do this then you need to map each field across manually.
Personally I avoid mass assignment. I prefer to be explicit in my code, and it also gives opportunity to give special treatment to individual columns as they are saved