Leeds fan here in peace.
Leeming is a good player and will have some great games, but he has his weaknesses.
He was more often than not taking the wrong option on important plays and isn't a 9 who distributes and works well with the half backs, largely this is the reason Rohan Smith preferred O'Conner (who is now a better hooker IMO!)
Leemings defence is pretty ropey for a nine, often pushed off in contact and can be a bit lazy in his defensive duties.
I think he's an OK signing for you guys but 4 years is way too long IMO and I would wager that e doesn't see out his contract just as he didn't for Hudds and Leeds, and he basically threw his toys out of the pram with us when it was obvious he didn't want to share the hooking duties and got dropped as captain.
From what Ive seen I think you've got a better player in O'Neil.