I have a Roomba 530, Neato XV-21, and Mint Plus. The latter two are new members of the house cleaning staff, under the direction of two cats.
Roomba 530 with AeroVac bin has the three-arm side brush, and that does well for sweeping certain kinds of debris into the sweeper path... things like spilled coffee grounds (oops, missed the compost bin, again). Some stuff that really light or just the right density, will get flung too far, but Roomba will probably pick it up eventually with its multiple random passes. Rommba's side brush was hit-or-miss on corners. The previous model I had with the two-arm side brush was made of a poorly chosen polymer for the brush arms. Soon after purchase, the brush arms were getting work-hardened and would break off. Fortunately, Costco/iRobot was great about sending out letters and free brush upgrades. The 530 is my third Roomba. It's not bad... just a bit noisy, and bonks around a lot. The exhaust skims the floor, which kicks up a lot of dust and fur into the air. I do like how it usually slows down before touching furniture, though not always (thin chair legs). Cleaning the CHM is better on the AeroVac bin, but it still is a pain, sometimes required mid-cleaning as the sweeper bars bog down with hair in the axels.
Neato doesn't have the side sweeper (at least in the US), but I do like the much improved suction and a single real beater bar that doesn't get tangled or bogged down with debris. The unit follows the wall, and lighter stuff, like dust bunnies and cat hair will get sucked in from the side. Heavier stuff, like spilled coffee grounds, no. The improved suction, drastically lower maintenance, and ability to clean my entire 1400 sq ft. bungalow without setting up virtual walls/lighthouses is a solid improvement over Roomba. The exhaust on Neato seems to angle upward slightly. It'll blow out corners like a leaf blower, but generally not kick up a plume of dust behind it like Roomba. I also like the dock - it pressed backwards against it, so less likely to push it around like Roomba sometimes does. When out cleaning the house, the dock has a much slimmer profile against the wall and the power brick is internal.
Just picked up a Mint Plus (BB&B using a 20% off email coupon). It definitely solves the corner problem, even better than a side brush. For light sweeping, it's like going over your floors with a white glove and a tack cloth - so long as you don't have a lot of debris, it'll get what the robo vacuums miss. For damp mopping, I find it quite effective with its patient scrubbing back-and-forth. Initially I tried a disposable Swiffer Wet Cloth, but the fragrance was cloying and overpowering, like cheap cologne... they need to make those in unscented. The included microfiber wet cloth with liquid dispenser solved that problem and cleaned deeper into the texture of my grungy 1980's vinyl floor. It actually did a better job than I do, short of getting a bucket of ammonia water and a deck brush out. I think its patient, even scrubbing and use of water only is the key. No residue. I also like how simple the Mint is... the fewest moving parts to do the job effectively is an advantage.
What I'd like to see, if anything, are a few basic improvements and a modular version of the Neato. It needs to protect its laser guidance turret cover better so it doesn't get stuck under certain conditions. The cover sticks up a good 10mm higher than the plane of scanning, sort of like Roomba's IR receiver button that can get stuck as well, though the Roomba has its IR button mounted on the bumper, which helps.
It would be great to have a mode for the Neato that turns off the vacuum/brush and could then use a wiping cloth like Mint. Even better would be a scrolling damp mopping cloth. It could do the whole house and just scroll the cloth slowly to expose new surface as it cleans... perhaps the scrolling would be driven by a gear reduction off the brush motor, since that is also variable speed. Brilliant! The Neato would be almost as quiet as the Mint in wiping mode.
So, side brush? Meh. Sure, if Neato Robotics can get it to work well, otherwise, there are other priorities like protecting the laser guidance turret cover. Hopefully they add some logic for a setting to prevent Neato from climbing up onto things. There's an accelerometer already in there that can very accurately measure tilt. Let me specify "No Climb" to keep it from going up onto things like lamp bases and the curved legs of my antique dining table. It's frustrating to watch it try over and over to climb up my dining room table legs.