Awesome. I'm glad you guys are getting excited about this because if the Celtics for Festus I would immediately go broke buying his jerseys. (It's EZELI, by the way).
Anyway, I run the Vanderbilt SBNation blog over at anchorofgold.com, so I've been following Fes throughout his career. I was actually just writing about what a great combine he had (34" flat footed vertical (6th best in camp), 7'6" wingspan (2nd best in camp), and speed similar to the wingmen when it came to sprints.
That's all very impressive because it's finally a reflection of what he can do when healthy. He broke out as a junior but had to play at about 80-85% throughout his senior year thanks to knee injuries. He tweaked one knee in the preseason, and when Vandy started out 6-4 he was pressed back into the rotation despite probably needing more rehab. This led to some issues with the other knee (overcompensation) but it's unlikely that it's anything permanent.
Still, he continued his reign as a top flight defender in 2012. He's a big, mobile center who is a strong help defender and shot blocker (see the wingspan/vertical combination above). He was very raw when he first arrived in Nashville (just one year of organized bball before going to college), so his first three seasons were difficult. The learning curve was very steep, and even after redshirting he was stuck behind All-SEC center A.J. Ogilvy. While he was a defensive beast, his offensive game was still pretty rudimentary, he fouled too much, and he couldn't make his free throws.
He made the adjustments to become a starter when Ogilvy left school, and it paid off in a big way. He learned how to use his strength in the post and developed a pair of moves that made him an offensive presence in the paint. He improved his free throw shooting to prevent opponents from hacking him once he had the ball. He learned to control his body well enough to avoid contact when leaving his feet.
It was a pretty impressive development from a guy who has had a NBA body since coming to Nashville. Plus, the guy just punishes rims like they owe him money. His dunks are equal parts angry and awesome.
Finally, he's a top notch person on top of that. Great work ethic, humble, polite, and willing to learn. Very coachable and understanding of the work that it takes to reconcile basketball knowledge to his natural abilities.
So that's Festus Ezeli in a nutshell. I think he'd be a great addition for the Celts, although there will be a learning curve for his adjustment to the big leagues. He's proven he could handle it in the past, and I'd be pretty confident that a guy like Clifford Ray could bring out his inner Perk.