Mechanics: Every time there's an election you choose to either stick with your current ruler, which gives him +1 to everything but costs you 10 republican tradition, or picking a new ruler, who will have 1 stat at 4 and the others at 0. High Tradition lowers your revolt risk, low tradition increases it. Falling below 50 tradition causes noble (ie reactionary) rebels to appear, falling below 20 may collapse you into a Republican Dictatorship.
Pros: No worrying about legitimacy, no more getting stuck with terrible leaders for ages, lowered RR with high republican tradition, Plutocracy idea group is pretty sweet
Cons: no more 6/6/6 leaders in your future, dealing with Republican Tradition means that either mediocre leaders or high RR are going to be a problem, no royal marriages (which also means no diplo vassalising)