It's an excellent alternative choice for In The Wake Of Gods modification, being the most appealing for those who don't want to chance the core-gameplay of Homm III much, but rather keep the changes very moderate.
The mod (or "an unofficial expansion", however you may call it) makes Homm III a better game pure and simple.
Horn Of The Abyss brings many new smaller yet clever upgrades and details to Homm III, re-balancing it, improving the game-interface, and adding many small things and tweaks to it on similar depth-level than previous official expansions of Homm III did at the time of their release. Heroes Of Might And Magic III: Horn Of The Abyss (HoTa) is a brilliant fan-made modification "from Russia with love" on the conceptual level aiming to be rather "an unofficial expansion" for Homm III than "a mod", being a direct spiritual sequel for the "Shadow Of Death expansion".