Firstly, please note, the score I gave is for the current (i.e. 3.0.1) version.

Stylish (up to 3.0.1) was a wonderful thing. You could style (that is, inject CSS into) websites (with various, like regexp, methods to define which urls to style), and, which is super important for me, you could style XUL (UI) in Firefox. Some may say, this can already in Firefox be achieved with userStyle/userChrome.css files in your profile dir, but stylish had a few advantages over those:
* you could split the rules into multiple parts and independently enable/disable those parts DYNAMICALLY while the browser is running (without even needing to reload the page or restart the browser)
* you could test rules dynamically and change them until you have what you want, again without restarting the browser or reloading the page
* you had a centralized system (userstyles.org) from which you can download rule sets and it's ultra simple (you don't even need to know anything about CSS)

Now, about stylish 3.0.1.
I don't really know the full set of changes it made. Some, or most of the advantages listed above might still hold. I don't know, because I have downgraded it immediately after restarting Firefox (after an upgrade) showed me the damage done.
What damage and why did I downgrade? Well, it's simple. The problem is XUL STYLING IS REMOVED.
XUL (Firefox UI) styling is what I mostly use stylish for, so without it it's basically useless to me.
Now, I know Mozilla has decided, against the wishes of many, to remove the ability to style XUL within the new web extensions API, which will be the only API for addons in Firefox 57 and beyond. And I know stylish developers wanted it to work in 57. But stylish 3.0.1 is advertised to work with versions from 48 to 57. So I would imagine, it should allow for XUL styling in 48-56 and only ignore it for 57. I think this is possible, as stylish could be a hybrid legacy/web extension addon like there are some.
Also, another thing is that stylish 3.0.1 doesn't tell you in any way on update "hey, some of your styles were disabled because of blah blah, here's the list: blah blah; if you want those to work, please downgrade". Nope, there's no such info, not even a "screw you". And I know it's possible to even display a new tab with info upon update, since NoScript does that for example.
So, in summary, my issues (could be possibly that there are more):
* My XUL styles stopped working, Firefox greeted me with it's default, (in my opinion) ugly, cramped and borderline unusable theme. It's possible all my scripts stopped working (including those for pages), but I didn't even check, as lack of XUL styling makes this version unusable for me.
* There was no message informing what happened and what I can do.

Here's what the stylish developers could to in order to fix those issues and increase my opinion:
* Include support for XUL styling fro Firefox pre-57, possibly by making a hybrid extension.
* If that's not acceptable, maybe fork stylish, with one fork for 57+ and another for pre-57 with XUL styling support, with both maintained.
* If you want to keep not supporting XUL styling:
* Stop advertising this extension as working with pre-57, because it's a lie - currently it's crippled in pre-56 to fit the requirements of 57+.
* Please note so clearly in the release notes, so one can click on release notes in updates in about:addons and see this update is unacceptable without wasting time by updating, figuring out what happened and downgrading. 99.9% of users won't read your website or github before updating; probably also nearly no one will read release notes, most people will get an auto-update, but some will and so such note would be helpfull.
* Also, please show a page upon update explaining what happened (some of your styles were blocked because X and the list of them is Y), what can you do to resolve this (e.g. downgrade) if something (e.g. blocking of some styles because of X). This will be very beneficial to those with problematic update, because they will: a) know what happened; b) know how to fix; c) be able to evaluate wether they can accept the update or have to downgrade. And it will probably for many significantly reduce wasted time (if an addon malfunctions due to an update, I can sometimes waste a few hours trying to figure out what happened before I find the culprit and downgrade, because upgrades are often done in bulk and you don't know which addon to blame, or is it maybe the combination of addons that causes this, or if it's a firefox version-addon version combo, or just firefox, etc etc. and also the issue can manifest itself weeks after the update, making the investigation even harder).
* I had something else to list, but I''m so furious and have been editing this comment for an hour, that I forgot. Sorry.

Anyway, if you ignore AMO comments, please let me know, so I can nag you on github or whatever.

AND NOW SOME NOTES TO USERS AFFECTED BY THIS TRAGEDY:
1. You can downgrade to the previous version, 2.1.1. On the addon page on AMO, there's a sidebar on the left with "More information". There's a link there "See all versions". Click it and in the opened page, find the version you want and click on "Add to Firefox".
2. In my case at least, after downgrading my styles were there. But in case you're afraid of loosing your data, best to backup the profile directory, or at least the stylish.sqlite file inside of it - it's an sqlite database file, which contains a table of all your styles (both enabled and disabled) with their ids, names, and content. If you think you lost the data after an upgrade and don't have a backup of this file, then don't panic and:
* Close Firefox.
* Copy stylish.sqlite somewhere.
* Start Firefox, downgrade stylish, restart, check if styles are present.
* If not, check if the database file (stylish.sqlite) you copied contains the style data. You can do that by loading the file with sqlite3 tool and issuing "select * from styles;". This can be done with "echo 'select * from styles;' | sqlite3 stylish.sqlite" on unix. If you have trouble, send me this file in an email and I'll check it. Don't worry, it doesn't contain any sensitive data. If the data is present, you can just create styles you want and copy-paste the css code into them.