I personally really like McDermott products and have both a G-core and an I-2 shaft for some of my cues. I have never used the I-3 though, so I can not comment on that particular product directly.
However, I can say that the I-2 reminds me a heck of a lot of the OB classic, with the similar diameter, taper, and 1/4" ferrule. The OB classic has better LD numbers and OB is also another company I love. There is something about having a McD shaft on a McD cue that I like for whatever reason.
Basically, I know this isn't the most informative thread, but I just wanted to say I like the I series shafts in general, McD products in general, and I hope you enjoy your new shaft. It all comes down to what you like and get used to, some like the highest percentage deflection numbers ...some enjoy middle of the road numbers ... some just can't get used to LD shafts at all and are used to compensating for deflection and do well at it. None of these approaches are wrong, it all comes down to what you personally like the feel of.... don't worry about the numbers!