I would say Kristic plays a lot like Pau Gasol in this video, with more limited skillset and speed. Can get in the open area in and around the rim (which Rondo will love). I think he is solid enough of a finisher that teams won't be able to bust out as much help-defense aginast KG in the low-post.
He is a solid rebounder, but lacks the toughness that Perk gave us......toughness though can be taught; look at Von Wafer. Since when was he known to be able to stay with his man? I think Kristic will give us equal production overall statistically at Center, but we will have Dwight Howard to worry about now. I think we could really use a healthy Shaq back in the mix, and a Troy Murphy buyout makes us a better team.
The talent that Jeff Green possesses is obvious, and I think he will flourish as a 6th man, and will improve drastically on the defensive end. Being around the big-3 (especially Pierce/KG), lays down the foundation for him to develop in to an All-Star a couple years down the road IMO.