What is it ?

The betweenlands is a mod adding a derelict swampy dimension between worlds;
It has pretty and cool visuals using custom shaders by default, tons of cool building blocks and particles, has ambience/sounds effects which are unique and add to the immersion, cool soundtrack (boss music included within the mod by itself) and an overall creepy atmosphere.
It contains lore which is left open to interpretation through small notes or implied by the existence of structures and creatures.

What does it offer ?

A brand-new yet familiar survival experience with a progression system fully independent from the overworld, featuring unique survival mechanics such as the decay , tool corrosion and food sickness , challenge through its tough enemies, dungeons and bosses and a both pretty and eerie ambience.
It provides its own take on minecraft's systems (its own farming , brewing and repairing system among a few others, and even some brand-new re-thought weather ); and it is not hesitant to rethink its past ideas to better fit with the newer and updated mod features.
It encourages exploration, having multiple biomes with unique fauna, flora, and global generation quirks which make each area feel different enough while still keeping that swampy vibe; it also brings in new ways to travel the world through items ( Tongue grappling hook ) and vehicles ( Rowboat and Draeton ), and adds new companions to take with you on your journey ( Emberlings , Harlequin Toads and more !).

The cons of this mod would normally be the performance aspect of the mod itself, as even though it is quite well optimized for a mod of its scale, it has by default configs about shaders which can be too much to handle for a basic laptop, but those visual effects can be reduced or even fully disabled in the configs and it works fine (I would recommend disabling them if your computer can't handle shaders or visual effects in general, it really makes a big difference).
It is also of note that this is in general a pretty difficult mod which is not recommended to play for people looking for a more chill adventure (even if it can still be calm at times), and its focus on exploration might not appeal to some tech modpack enjoyers either, as most systems do require human intervention/maintenance and can't be automated forever (there are unofficial addons for that though, but that isn't the subject of this review).

In conclusion

It provides a haunting medieval swamp urbex experience, with a pretty yet dangerous environment which has content encouraging exploration.
Most things stay cohesive within the mod, as updates of the mod take into account what has been and what will be.
It is well documented as it has advancements hinting at the various areas, features and systems and an official detailed and up-to-date wiki, and that's not even counting all the little things such as how the mod tells you in which special block an item can be used in.
Finally, it has also plenty of niche details and hidden mob interactions, leaving the world full of surprises, to the point where I still discover mechanics and stuff about the mod, even though I played it for more than a couple hundred of hours at least (probably more).

I would like to add that this mod being my absolute favorite, its review might be somewhat biased, which is why I urge you to try it for yourself, after all, you might still like it despite its difficulty.

Looking for addons or extra content ? on a given moddex mod page going to the "Relations" tab then setting relation types to "Required" + selecting "Dependents" instead of dependencies then Project type "Mod" shows you every mod which requires this mod to function (which should correspond to known addons)