My experience with BatteryUgrade was a positive one. They were helpful and responded promptly to my email query pre-purchase. Delivery was prompt too.
About the product:
My phone is a Samsung Galaxy 4 Mini, ie quite old. BatteryUpgrade's website was easy to use and offered several suitable replacement batteries, I chose one with a 50% higher capacity spec than the original. It is performing well, it charges fast and the phone now has much longer battery life than it had before. Thanks BatteryUpgrade!