I think it took me about 10 minutes to get used to it, it really is rather convenient. I still use <enter> very often when typing regular commands, but when I cycle through previous commands with ctrp+p/n it is much easier to hit ctrl+j.

Also, because my hands stay on the regular keys I don't have to look at my keyboard when switching betweeen typing commands and going through the history. It just feels more natural.