Cydia 1.1.20 fixes a number of issues introduced in Cydia 1.1.19 (which was a massive change to Cydia's internals).
Cydia 1.1.21 fixes some more issues introduced in Cydia 1.1.19 (oh well).
Cydia 1.1.22 fixes an issue introduced in Cydia 1.1.21 (this is not going well).
Cydia 1.1.24 fixes some bugs and is designed for the new iOS 9 jailbreak.
Cydia 1.1.25 is a single modification designed to quickly push for iOS 9.
Cydia 1.1.26 is a single modification to increase the reliability of 1.1.25.
Cydia 1.1.27 is a single modification that makes Cydia work better with stash.
Changes in Cydia 1.1.19
Cydia Substrate Compatibility
Cydia now runs as "mobile" instead of "root". This means that Cydia Substrate extensions (tweaks) can now modify Cydia, just like those tweaks modify other apps and iOS.
How this will affect you: After you install this upgrade, you may be surprised to see that your installed tweaks are modifying Cydia. For example, if you have a tweak installed that improves how the keyboard works in all apps, it will now improve keyboards in Cydia as well.
Tweaks may have problems: Some tweaks may have bugs in how they affect Cydia, such as applying an odd color to a piece of text, because their developers have not been able to test this before. Please be patient and give developers time to update their tweaks — you can often follow them on Twitter for news and updates. You can temporarily uninstall a tweak if it is causing problems for you.
What if something goes wrong? If you install a tweak that causes Cydia to be unusable, you can fix it by booting your device with Cydia Substrate disabled (which also disables all tweaks). To do this: turn off your device, and turn it on while pressing the volume up button until it finishes booting. Then open Cydia, uninstall the buggy tweak, and reboot again to go back into normal mode.