Upgraded from my Paperwhite (the latest pre-waterproof one). I had tried the newest Paperwhite but was put off by poor text contrast/blurry text, and a lack of low brightness settings. The Oasis mainly fixes those complaints, but it's not perfect.
Firstly, I bought this mainly for the adjustable warm backlight - this is a lifesaver for me as almost all of my reading is done right before bed. I can already feel the difference this makes; the paperwhite's lighting was always much better than a phone or tablet for not being backlit, but the warm light on the oasis is a leap beyond that and leaves me feeling noticeably more tired after reading and lets me fall asleep faster. This is a fantastic feature that works as you'd expect. Another step up is how incredibly fast it is to turn pages and generally use the interface - this has seen improvements with every kindle and the Oasis is the first one where the menus aren't laborious to use. And page turns are blink-and-you'll-miss-it fast.
However, I actually find almost everything else about the oasis to be a mixed change, or a step down, from my paperwhite. For one, I was looking forward to the pagepress buttons but compared to my 3rd generation Kindle with keyboard, the buttons are way too loud (they click very audibly when pressed) and too small - the original kindles had a very tall page forward button and a small page back button, which was better. I find myself using the touchscreen to change pages more often than using the buttons, so I would have preferred that they be absent. Also, much like the new paperwhite, the display doesn't go very dim; I used to read at a 4 on my old paperwhite, I found the newer paperwhite's 1 is about as bright as a 6 or 7 on my old one which was a dealbreaker, and the 1 on my oasis is somewhere around a 4.5 on my old paperwhite; usable, and I find the warmer light makes it feel dimmer, so this is fine.
There's also the asymmetrical body of the oasis - I read reviews saying people like it and find it easier to hold, but I feel very strongly the opposite. The oasis is significantly more uncomfortable to hold than my paperwhite. I guess part of it is the larger size. I do enjoy the larger screen and I can notice that each page contains a bit more text, which makes you need to turn pages less often, but in retrospect the 6 inch display of the paperwhite was better.
Finally, the flush-front display is nice for being easier to clean than the recessed-display paperwhite, but it shows a ridiculous amount of glare (e.g. can't have my casper glow light on even at a low brightness while reading) and seems to get dirty/fingerprinted much more easily.
Overall, I still think this is the best kindle for nighttime readers because of the warm light, which is a fantastic feature, but there are lots of tradeoffs (unpleasant buttons, lack of dim brightness levels, hard to hold, screen gets dirty and shows glare).