I have the S300i - I just got mine last year after having the Hayward Navigator before which I was happy with as well. We don't have trees in our small backyard so only get leaves when we have a wind storm several times a year and dirt when it rains which is usually 5-10 days a year.
Here are my thoughts... I had a leaf basket on my Hayward and so it was great for picking up leaves. With the Robot, it will pick up leaves but like SRCTulsa mentioned if you have a ton of leaves and things you'd probably want to use a net to get as many as you can out before running the robot, leaving it to focus on the finer cleaning mostly.
The robot is WAY bigger and WAY heavier than the Navigator, especially when pulling it out of the water because it holds water and all the stuff it's picked up. I can definitely see people throwing out their backs pulling it out of the water - I was a bit surprised by that myself.
The robot is NOT any smarter than the navigator, at least not in my pool. That may be different in a more normally shaped pool - our is very random so the robot much like the navigator just seems to run in random patterns.
The robot does a great job of "scrubbing" the floors, walls, and tile lines with it's rubber brushes. Mine does get up onto our baja benches and stairs as well, though it's not smart enough to know it's up there so it's not like it goes up there and cleans the whole bench so I do have to brush the benches/stairs. I use the robot usually once a week and run it for two cycles. Besides picking up all the dirt/debris on the pool floor and scrubbing the walls, it also does a great job filtering the water so it's cleaning the water itself while it does it's thing. The "tangle free" cable is a bit misleading - the cable has never gotten tangled in a way that interferes with the robot during operation so that is certainly true - but it does somewhat wind itself, so after every 3-4 uses I have to detach it from the programming unit and unwind it. The robot I got came with the Caddy and I have found it to be very helpful for my use since I store the robot in the garage and just wheel out the caddy when I'm ready to use it.
I think that about covers it. If you've poked around the forums, you've probably already seen that you should look to buy one from Marina Pool Spa & Patio - they really do have the absolute best prices by a long shot. That's where I got mine from. Also, my first one was faulty so I did have to deal with Maytronics and get it replaced, but their customer service was pretty good (other than long hold time to talk to someone) and they of course covered the cost of shipping mine back to them. Once I got a non-faulty one, I have had no issues. I don't think I could ever go back to the suction Navigator after having the robot - while it was great, the robot just does a more thorough cleaning than a vacuum ever could.