I've said it many times before in this thread and in the general CL thread, this won't be easy. I saw Celtic live last year when they played Helsingborg in the CL qualification and they weren't having a good game but still won (0-2). Then they beat Helsingborg at Celtic Park for the return leg, 2-0. Can't say I've watched them much more than that recently, just the Barcelona highlights. Their home record is impressive and their fans will lift them.
I think they have a few great players. Hooper impressed me as well as Samaras (I know he's criminally underrated by some). Samaras might not be very quick or technical but he's very dangerous on set pieces so we better watch out there.