I've just finished reading Mythic Rome and I have to say it's a wonderful piece of work. I hadn't read the BRP Rome, Life and Death of the Republic book, Pete's previous effort on the subject so I can't offer any comparisons.
The background information is fantastic and in many places split into the various periods of kingdom, early, middle and late Republic so you can tailor your games accordingly. The character generation section follows the usual Mythras style being tailored to the period and providing great backgrounds and hooks for gaming.
The section on the various types of Magic usable in Republican Rome is a masterpiece of game design with helpful systems that give a great feel to each and every one of those types. I'd recommend getting the pdf just to crib those for any campaign that has any level of magic use from not really to fairly full on. It's never flashy magic often more psychological than direct and better for that as you can never tell whether someone has real skill or it's smoke and mirrors backed up by playing on your fears.
It has inspired me to want to run a game of political intrigue set in the First Century BCE where the various civil wars form the backdrop to fierce political struggles in Rome itself.