(Some spoilers ahead)
The fact that the time travelling wasn't explained too much didn't actually bother me too much, but I think the anime could be better if it explained how the time travelling worked. It was mainly the ending that left me unsatisfied... Part of it is purely personal, since I wanted Satoru to be with Kayo. The other part of it was that Satoru lost 15 years of his life in the process of trying to save people and what did he get? Lost time and missed opportunities... He didn't deserve that to happen to him... Even though that might be a "realistic" type of ending, it just doesn't sit well with me, so I guess both reasons I disliked the ending were personal...
Well to conclude, the anime is good, very good. It's just that the ending that is not satisfying... It was so close... so close to being perfect...
