I've really enjoyed my mi3 so far, the battery life has been excellent and could easily get 3 days of light usage or about a day and a half of heavy usage, by the time the battery is getting bad you would want a new phone anyway, for example we had a iPhone 3g with sealed battery, it came out in 2008 so its 6 years old the battery life is bad but still use able, what phone would you wanna keep 5-6 years anyway. also I've owned a few phones with sealed batteries that were more then 2 years old and were previously used, battery life was good, so it will more then last the usefulness of the phone.
if you need space you can get a 64gb version, the mi3 is very cheap now about 275 USD, you can't buy any snapdragon 800 phone for that, I don't regret buying my mi3, plus with miui v6 round the corner it just gets better. And if you want to shell out the extra cash the mi4 is a nice phone too.