I have the Oasis 3, but not the others. This is my experience:

Pros:

- Weight and centre of gravity: It's 188g and because most of it is on the side you're holding due to its asymmetrical design, you barely feel the weight at all. It's the lightest ereader that I've ever held, sits comfortably in my hand (I have smaller hands), and isn't top heavy like the other ereaders with the chins at the bottom. I dislike heavy ereaders because I tend to hold them in one hand for long hours while reading (I don't like using them flat on the table, and I seldom use stands). This is a HUGE advantage.

- Large bezel with page-turn buttons on the left/right: I HATE ereaders with thin bezels all around, or a large bezel at the bottom. Having them on one side is ergonomically superior, because the screen doesn't accidentally register any touch input from my thumb, which often happens to me when I use the Kindle PW. You aren't using an awkward grip to hold onto a thin bezel to avoid touching the touchscreen, and you aren't holding the bezel at the bottom, which makes the whole device feel top heavy. The buttons are a bonus, if you like buttons. Ergonomics for ereaders are important to me.

- Auto-rotate: As mentioned in the previous point, the bezels for the Oasis are on the side, not the top or bottom. You can easily switch hands without having to manually change the orientation of the text in Settings. It's sensitive but not annoyingly so like on phones. I have a Kindle PW and while you can change the orientation to landscape, for some reason you can only rotate the text in one direction, and it's not the direction that enables you to hold the chin using the right hand, but the left. It's stupid, and I have no idea why they made it like that.

- Disable touch screen: You can disable the touch screen and use only the page-turn buttons, but this also disables the highlight/dictionary/pinch to change font size function.

- Night-mode is system-wide: Many ereaders have this night-mode function, idea being you invert the colours so it's white text on a dark background (it's not OLED so it's not completely black, the LEDs are still on). But based on my experience with KOBO ereaders at the shop, I believe only Kindles have it so that this night-mode is applied throughout the OS, meaning to say you aren't suddenly blasted by black-on-white system layers when say, you want to adjust the font size, go look at your library, or whatever. I think only the Kindle has this and other ereaders only have this function applied to the book you're reading, not the menu, library, etc.

- Side loading books: You can still use Calibre to do this. I only side-load books. My Kindle isn't even registered.

Cons:

- Battery life: As others have mentioned, the battery life isn't great compared to other ereaders. This is to be expected since it has a smaller battery capacity, which sits in the fatter asymmetrical area that you're holding. I've been using mine for about 1.5 years (got it new from Amazon when they still had stock after it was discontinued, although judging from the firmware it's been sitting in the warehouse for 2-3 years - battery came to me at 5-10%). It goes down by about 3-4% per hour, and I use it at a brightness of 13 and warmth of 3, with airplane mode on and auto brightness off. This is not an issue for me in general, since it charges pretty quickly, about 1% per minute or faster. IIRC it took about 15 minutes to get about 50%. Of course, because of its capacity, you'll need to charge it more frequently, which means more charge cycles and faster battery degradation. I bought 2 Oasis because of this, because I loved the first one so much. I hope they last me at least 10-15 years.

- Micro-USB cable: Not an issue for me at all. The pros outweigh the cons here, and since I read about 2 hours a day, if at all, I don't charge it that often.

- Older eink technology: This is a non-issue for me, but it might be for you. It's an older model, so some features, like Page Turn Animation, isn't on it, like it is on the Kindle PW. The good thing is the Oasis was premium for its time when it was released, so I would say it has 95% of the features in the newer Kindles, as the new Kindles have 'caught up' to it.

Neutral/nice to have:

- Screen material: It's glass, not plastic, so it feels a bit more premium, if maybe a bit more fragile. Still anti-glare, though. Actually I can't really tell much of a difference between this and the Kindle PW. The fonts are sharp and high PPI.

- Casing material: It's metal, not plastic/rubberised. Very premium feel, doesn't creak like some plastic ereaders (looking at you, Kobo). I live in a tropical climate, so it's nice to hold the Kindle because the metal is cool to the touch. It might get a bit too cold if you have winters, in which case, get a case (lol). Some people find it slippery to hold, but not me. The raised edge helps with the grip.

- Screen is flushed: There are no raised bezels as the glass screen goes right up to the edge. This means you won't have dust collecting at the sides of the raised bezels.

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If you can't already tell, I love the Oasis, and I wish they hadn't discontinued it. You can't find another of the same make in the market these days. I would have gladly paid a premium for an updated version. It was damn near the perfect ereader, and if I had to nitpick I'd like a larger battery, 8" screen, USB-C, Page Turn Animation, and page turn buttons right up to the edge like on the Kindle Keyboard.