I'm a big fan of 5/3/1, and I was pretty happy to see an app finally! It was a little confusing at first--a good page of directions would improve the value the significantly, at least at first.
I would like to see improvement in the "assistance work" area--first, the ability to actually specify what exercise I'm doing, then the ability to track a full a workout (here are my sets of Deadlift, my sets of pull-ups, and my sets of shrugs, performed on this date).
As it is, while this can replace a spreadsheet program (or just a bit of math in your head), you'll still have to keep a separate journal.