The problem with Acorn is the monthly fee. If you really are just investing small amounts of money each month (maybe $20-40,) that dollar may very well be most of your profit for that month (or very likely, more.)
So, if you are going to use their service, you will need an initial investment that is large enough to hopefully offset the fee. They are not actively managing the investments or anything, all of their options are index funds, so the fee is only for the convenience factor of using their app. Their options are limited and they offer very little in the way of advice/other features.
IMO you would be better off doing it yourself (the rounding thing is a cute idea, but not worth the fee.)