It was terrible because the "managers" had very little training which made them mostly useless and a legal liability to the company in regards to employment law cases. In many instances they weren't even on the same direct team but an adjacent team, so rhey hahd very little interaction. This completely invalidated the premise that a technical/coding manager would be a better mentor since there was never any time for it. Of course the company paid them the same rate as the senior devs that weren't managers. I'd say at least 50% of the first year cadre left the company or reverted to a regular senior dev after one year or less. Most divisions of the company don't use this model now. The only real reason they did it was because Google did it.