I have great first-hand experience!
Got my brand new i3 less than a week ago and yesterday I needed to go pick up some ducks. 20 loud, stinky, muddy, poopy ducks. Folded back seats down, ensured the 'i function' cover was well-placed to cover as much as possible, and then put 4 cardboard boxes in the back, each containing 5 ducks. Journey home was uneventful. When I was unloading the 4th box, my dog picked up on me having something potentially tasty to play with so jumped up, knocking the box from my hand. One of the ducks popped out of the box and went flapping into the car with muddy, shitty feet as horror and grimace rinsed across my face.
So I mentioned the dog. She's a big dog with a double-coat made for surviving arctic cold--an Alaskan Malamute. Been transporting her all week in the back (seats up) with the 'i function' cover. She is currently molting--I could knit a sweater with how much fur I brush out of her every evening.
All of that info to set the scene of how I spent an hour cleaning my brand new, less-than-400 miles on the clock, i3 from mud, duck shit, and dog fur. Like all things, the right tools make a job easier. We have an animal model Dyson--the animal brush attachment sucks up fur from any material without issue. We've been using it for a year in the house so already knew its value--same in the car, perfectly vacuuming off the dog hair from all the various surfaces/materials. I mention the Dyson first because it came with a foam cleaner. I tested a small spot to make sure it wouldn't discolor the fabric. It didn't, and 30 minutes later I was vacuuming up the dried foam right along with the dried mud and duck crap. For the non-fabric surfaces, I just used a generic 'all surface' cleaner--spray and wipe. The "hardest" part, which really was not that hard at all, is the slightly raised hex pattern of the plastic. It required a few more circular movements to get the mud/crap off as opposed to a perfectly smooth surface but again it wasn't hard at all.
Everything is back to brand new and was not difficult to clean. I don't intend on mucky ducks flapping around the car again, though the weekly vacuuming of the dog hair will be a quick 5 minute job.