I was working in the John Smith's in a sports role while he was working at Town, so I saw a lot of football there during his tenure. In the club, there wasn't anyone who said anything other than him being a top, top manager. The work at Huddersfield was excellent. He had a really mediocre squad, a very precarious ownership, and a lot of turbulence at the club and managed to make Huddersfield one of the hardest teams to play against and beat in the division. Getting to a playoff final with that squad deserved a medal by itself, and he was one dodgy penalty decision against Forest (who have the cheek to talk about refereeing decisions) away from being a Premier League manager.
At West Brom, up until recently he's faced a similar situation, come out the other end, and his team now looks one of the most effective, cohesive, and exciting in the league. Two goals conceded in six games. At Olympiacos, he was set up to fail and made some mistakes, but everyone makes mistakes. It's a blot on his copy book, but doesn't seem to have stopped his momentum.
Plays some really decent football as well although despite his Bielsa connections, it isn't the same. Lots of baiting the opposition with slow build-up and breaking at pace, lot of creative set piece routines as well. While he used a load of different shapes at Huddersfield, pretty much setting up to stifle the opposition, he seems to have used a 4-2-3-1 everywhere else. Town played 3-4-3, 3-5-2, 4-3-3, and 4-4-2, sometimes in consecutive weeks.
I remember a journalist who covered Town and was a good mate of Chris Wilder, who said that Wilder told him he had been preparing for Town to play in a certain way, and when they kicked off they were able to spring a completely different plan on Boro that they couldn't match - Town won that game 2-0 away, by the way, apparently he described it as "one of the hardest games he's ever managed."
Everton would be really, really lucky to get him, and he has been used to pulling sides that are out of kilter into shape and making poor players better by building a really cohesive gameplan. Sound like it might be a decent fit for me.