I have owned an I-4 for half a year or more, I absolutely love it. The negative posts that pull back to allow your battery in the charge slots have recently begun to "bend" slowly back which would make charging harder due to a poor connection... They are easy to push back with a pair of pliers back to their original position, no problem.
They charge my batteries great, and once a few batteries are full it focuses all its energy on the cells that are not, making it nice when you are counting on a pair to be ready, or all of them. I was on a small budget when I bought the I-4 and I wanted the D-4, however I am very happy with my I-4 and might end up getting a D4 at some point, however not a thing wrong with either.