My wife's best friend has an iphone 6... After 2+ years, she would charge it for the night and when she woke up, battery displayed 100%. 12 minutes later, it would shut off. If she didn't use it at all, it would last 30 minutes. She constantly had to keep a large external battery pack with her to keep it from dying. I've used iFixit for many repairs in the past and told her that I could put a new battery in it for a lot less than buying a new phone... or even a lot less than having Apple to the repair. I ordered the battery, she stopped by the day after it arrived and I installed it in about 15 minutes. It would have only been 5 - 7 minutes but I needed to find some way to tape the battery down (forgot to order replacement tape strips). Anyway, she is now able to go hours without having to charge the battery. On really heavy use, she can still kill it in about 2 hours... with limited use, 6-12 hours... sooooooo much better than 12 minutes of limited use.
Easy. The video on youtube was easy to follow and as long as you have the correct tools, it was a snap. It's literally about a 5 minute job if you have everything right there. It took me 15 cause I had to look for replacement adhesive.
DIY is so much more cost effective and can be easy with the correct knowledge and video demonstrations. iFixit is amazing.