As Coasterforce's resident I305 fanboy, here's my take:
The ride is stupid intense. It's relatively uncommon for most people to go through the ride without greying out at least a little bit.. if not the first time, then on a re-ride. By the nature of its layout, every twist and turn is taken at very high speed, which puts a lot of sustained positive G's on you. There are a few patches of airtime, and they aren't bad either (absurd trim brake notwithstanding), but I305 is really all about 3 things: a huge drop, sustained high speed, and positive G's. The ride is designed to suggest the experience of driving a race car, and I think it does that well.
With that in mind, it's not at all hard to understand why this type of ride doesn't have universal appeal. Enthusiasts aren't the only people who complain about its intensity. Several of my friends don't enjoy riding it, and neither does my brother, who will ride anything with me when we go to parks as a family. Some say positive G's are exhausting (they are, but I find them thrilling), some people don't like the experience of greying out (I kind of do), some hate the quick transitions (I love 'em). You need look no further than the fact that, on any given day, Volcano, and sometimes Dominator, will have longer lines than I305 - which at that height, speed, and cost should be the park's marquee coaster - for evidence that the ride is flawed.
That being said, it's also highly appealing to a certain sector of riders. To me, it's almost the perfect coaster when the trim isn't grabbing like it does this season. As it currently rides, it's still a ton of fun. It's an entirely different ride from Millennium Force, and I would say it's better, but I understand the opposite point of view.
Perhaps, one might say that I305 is like heavy dubstep: very high in energy and intensity, and equally appealing to some as it is unappealing to others.