Here are some reviews from TechRadar:
Android 4.4 KitKat review
FOR
Fast and frictionless
New productivity options
Improved Google Now
AGAINST
Hopped into bed with Nestle
No split-screen apps
Android KitKat started out as version 4.4, but it's since had various small updates in the form of 4.4.1, 4.4.2, 4.4.3 and most recently Android 4.4.4 - this review has been updated to reflect the tweaks and changes experienced at each step to give you the most complete overview of the operating system, as well as highlighting how things will change in the move to Android 5.0 Lollipop.
Android Jelly Bean review
Android 4.1 and then Android 4.2 has made Jelly Bean a thing of beauty. You'll definitely want to give Jelly Bean a look.
FOR
Silky-smooth performance
Improved notifications
Chrome as default
Resizable widgets
AGAINST
Google Now is limited
Slow roll out
Few tablet-friendly apps
No built-in lost device security
Android 5.0 Lollipop review
Android Lollipop isn't been great news for low-end phones, in which it appears to have incited raft of annoying performance issues, but the new look and feel are great. Lollipop marks a slightly different direction for Android, and it's a sensible one.
FOR
Nice visual design
Consistent aesthetics
Low-key 'digital assistant'
AGAINST
Slower on budget devices
Camera app underpowered
Android 5.1 still didn't radically change the system, but it did add a few features like HD video calling, a security tweak that blocks a phone even when it's reset, and a tweak to how the notification menu settings toggles work. The most important change, though, was to how the volume buttons work.