That's funny, my local brick and mortar convinced me to go with the S300 (I guess they push what they know - or probably what they have in excess inventory). It has exceeded my expectations so far, but with the poor performance of my old Diagnostic it had small shoes to fill. The basket is 1000% better than the old bag system and has much greater capacity than the cartridges. The whole unit seems to be a simplified version of the Dolphin platform, but not in a bad way. It's lighter, more powerful, climbs better, holds more debris and cleans more easily. My only gripe so far is the inconsistency of the Bluetooth connection. Don't yet know if that's just me or if it's the way the tech is. All you "lose" with Bluetooth disconnected is the digital "joystick" manual override, everything else can be controlled from the power supply. I'm confident that I would have been equally happy with the M400 (but now that I've used the basket I'm not sure if I can switch).
One other thing, to address the whole "worth the money" issue. When we bought the house a few years ago and became first-time pool owners, the previous owner gave me a quick tutorial on using the skimmer-attached vacuum. I did that for a month, along with periodic wall scrubbing. I then purchased a well-cared-for (but nearly 10 year-old) Diagnostic from a friend and it was a revelation. I really only have weekends to get stuff done because I work late most days, so I could drop the robot in, run errands and return to a clean pool. There was still about 5 minutes of skimming the surface with a net for little floaty things, but totally manageable.
The new S300 takes the convenience to the next level because my post-pool-cleaning ritual has been further simplified. The Diagnostic's bag system was pretty poorly conceived and even when we went to disposable bags last season, it was an imperfect alternative. It still clogged easily. So I have 3 years of in-warranty coverage, which makes the worst case about $350 a year for the convenience. My pool guy wants $125 a week for maintenance, so that's a no-brainer.
If it lasts as long as the Diagnostic, then it's under $100/year. Either way I have gained 18-36 hours a season, at a minimum, so I think it's well worth it.
