I would have loved to give Proffer 5-star as it is the only app that I know of that actually automates Buy-One-Get-One 50% (or any other percentage). But the response time after a stellar first support call was quite frankly, non-responsive. Critical as we launched our store with the advertised promotion.
UPDATE: If you book a call via the ShopTrade website, they will get any issues fixed. I would recommend them improving email support as it can be hard to get in touch to start with. The app is great though!
This app has a lot more features than any of its competition for the same price. The live chat support is also amazing at helping you shift over from other apps.