How helpful is LOMS for preparation?

Highly recommended. When I was near the middle of my own prep, I realized I wasn't making repeatable progress so decided to purchase LOMS. Years later, I am still convinced this program is what go me into BCG: listening to it on repeat until I knew what the candidates were about to say really helped me visualize what 'good looked like'.

After listening to the whole program nearly 3 times, I did other practice cases with BCG friends, who - to a man - pointed out I was a different candidate. From someone who'd crack a case every once in a while (and mostly through luck), I had become someone who could crack a case in a structured and repeatable fashion. From someone who could hope to get to a second round on a good day, I became someone who could reasonably expect to get an offer on an off day.

tl;dr: yes, purchase LOMS. Just like the excellent material available on PrepLounge or feedback from current/former consultants however, it will only do you good if you actually use it.

Good luck!