TL;DR: Advanced Labyrinth Lord is convenient and saves you the trouble of having to flip between the core rulebook and the Advanced Edition Companion book when running LL using the "advanced" rules. There's nothing new content-wise, Goblinoid Games is selling you LL + AEC condensed into one book which is a little disappointing.
Full Review: Advanced Labyrinth Lord is basically just the core Labyrinth Lord and Advanced Edition Companion rulebooks mashed into one single, easy-to-use tome. Honestly I can't help but wonder why Goblinoid didn't publish the game in this format to begin with instead of separating them into two different books. Or at least include the essential rules such as exploration, monster stats, and combat tables etc within the AEC so people didn't have to switch between the two books in order to play an "advanced" game of LL. I'm sure looking back Goblinoid Games realized that people would've much rather preferred to have just one book "to rule them all" which is why we got Advanced Labyrinth Lord.
As for new content, you won't find anything in Advanced Labyrinth Lord that isn't already present in LL or the AEC, and honestly I was a little disappointed which is why I took off a star. If you already own LL + AEC (which are free to download here on DTRPG) then you aren't getting anything new, just the convenience of not having to jump between two books when running an "advanced" game of LL. I felt like there could've been SOMETHING new that they could've added to make purchasing this book more worthwhile, such as maybe a few new classes (Barbarian and Sorcerer anyone?) or maybe some new player races/monsters/magic items etc. Instead, Advanced Labyrinth Lord's main selling point is just the convenience of having everything you need in a single book.
Gameplay-wise, Advanced Labyrinth Lord plays exactly the way you remember it. You'll roll 3d6 in order for stats and pick a race, then a class, roll for HP, then buy equipment and send your poor PC to an almost certain doom inside a dangerous labyrinth deep beneath the Earth. As far as layout goes, the book does a good job at separating the "basic" and "advanced" elements into their own sections, and carefully explains which sections to use depending on if you want a basic game or an advanced game. The "Advanced Hit Dice" option is still present, just in case you want your PCs to be a little more robust with their Hit Points. I felt like the book did a good job at explaining and making it clear which parts to use depending on if the GM is doing a basic or advanced game, however anyone not familiar with the differences between B/X and AD&D may want to carefully read the rules until they start to understand what the differences are.
Final Verdict: I would highly recommend this book to fans of LL and anyone wanting to run an old school AD&D game without having to try and decipher the mess of rules that is the 1st edition AD&D game (especially the "High Gygaxian Speech"). If you're happy playing just basic LL then you don't need this new book at all. Alternatively, if you're tired of carrying around both LL and AEC and want just one book with the option to play either basic or advanced rules then this is also for you. Lack of new content is disappointing given we're shelling out money yet again for a product that we already own, however it's not a deal breaker either.