In terms of my personal well-being, it was a godsend. As my dad tried to explain to me when attempting to talk me out of transition, a man (and the part he left out: with a male brain) who takes estrogen will feel more tired, depressed, and just not good. He cited my grandfather, a very masculine individual, who had to take medicine that raised his E levels because of prostate cancer.
Well, compared to how I feel now, that's how my entire life felt before. I've always been a very intelligent person. Even though we didn't officially have one, I was salutatorian of my high school class. I've been through college and have a good job. But never in my life did I actually care about anything. I had no will to live, no ambition, no hopes, and no dreams. I just went along with what everyone else wanted for me. Estrogen flipped a switch in my head and now I want to do so many things I will probably never finish them all. I guess you could say it made my life worth living again at a time when I was seriously thinking about suicide.
Energy-wise, I don't feel any different. I know that I'm weaker because my muscles are squishier if I try to squeeze them, but it's not something you notice until you can no longer lift something that you could before. I don't mind though, there's plenty of nice guys willing to carry things for us skinny girls

Overdosing on any hormone is bad. I'm sure you've probably read about what can happen to people on steroids - that's like overdosing on testosterone. I don't know the specific effects for estrogen, but they're not good. It probably raises the risk of breast cancer significantly (among other things). That's why we have doctors monitoring our levels regularly until we get on a steady dosage.