We have 1 6.4 - its been a pretty good truck so far but the rad issues (leaking) are a bit scary. Haven't had too much go wrong - 1 trip to the dealer for a CEL which resulted in a reflash which seems to have fixed it but the MPG is worse than the 7.3's pulling the same load. We take the 7.3's on the longer trips even though the 6.4 is newer.