He finished ranked 5 times at Texas Tech which is really good. But no conference championships, no conference championship appearances, no BCS bowl games, no top 10 finishes while he was there in 10 seasons.
Moving on to Washington State he finished ranked once in 8 seasons, but it was a top 10 finish. But once again, no conference championships, no conference championship appearances, and no BCS/NY6 bowl games.
He will get a recruiting bump by going to Mississippi St, but that will be negated playing teams that recruit far better than the Pac 12 north. 10 win seasons will be very unlikely for Leach in Starkville. Playoffs are out of the question.