This is the smallest controller I could find for the simple wearables I've been making, and it works great with all my Arduino sketches. They have every accessory you'd need for simple 3.7 V power. Easy breezy and low cost. My customers love the low price. I can't say much more besides that - nice, small, easy to use, and best of all, half the price of a Nano. Load the sketch, plug in a battery, and away you go.