The Aura ONE is a nice unit. It's bigger than the competition (the screen is a 7.8 inch Carta E.ink display with a resolution of 1872 x 1404 at 300ppi). It weighs 226g. So it's about the same size as a paperback book and weighs about the same (195.1 x 138.5 x 6.9 mm). It charges via microUSB and battery life is up to a month (varying by usage of course). The display is backlit and it has a light sensor which will adjust the colour of the front light to suit ambient surroundings (and time of day, so it will reduce the blue content at night - which can potentially affect sleeping technology called ComfortLight PRO).
Several standard book formats are supported, including the most popular ePUP, ePUP3, PDF and MOBI. It can also display several image formats and comic book formats (CBZ and CBR).
The main new feature however is the water resistance, it can survive getting wet (IPX8 i.e. an hour in up to 2 meters of water). If reading in the bath is your fancy, this is the reader for you (or even in the shower, though that's less practical). It's probably usable on a beach too, though salt-water tends to mess things up really badly if it does get into the electronics.
When plugged into a Mac/PC it appears as an external USB disk and books can just be dropped on to the device. Once unmounted (ejected) the books are 'processed' and put into the library ready for reading. Trying both a ePub and MOBI version of the same book (O'Reilly tech book), though the Kobo would read both, the MOBI version caused the Kobi to become very sluggish and reacting to page changes or going back 'home' took a while. The ePub version reacted quickly without problems. PDF's were sluggish too and caused various bits of the screen to flash as pages loaded and moved between bits of the screen.
If you have an account on the Kobo store, it's easy to download a book, just find the one you want, tap on it and it will download.
It's also possible to borrow books from your local library (assuming there's still any left in your area) using the Overdrive service. All you need is a library card and books can be requested. You'll be warned (3 days) when the book needs to be returned, then you can re-request it if you haven't finished it. The list of libraries is available here.
Assuming you stick to ePubs it's not a bad eReader and being waterproof is a nice feature. It's a shame MOBI/PDF aren't handled better.
The Kobo Aura ONE is available on-line for £189.99 which is a lot cheaper the Amazon's top of the range eReader, but a lot more expensive than their basic ones, though it's bigger and more book like. Oddly the Kobo site doesn't have availability at the moment.