I like to collect but don't obsess. So it's easy for me to ignore them. I typically enjoy them most in open-world games, because the collectibles are a good reason to go exploring. Most of the time, they could be better objects. ME3's upgrades, for instance, never made sense--why would these upgrades just be laying around? I like it when they are objects that tell little stories that flesh out the world, because those gamers more likely to go on treasure hunts are often more interested in backstory.
If the objects are going to be a benefit, it ought to be unusual and convenient--but unnecessary. I liked the Fallout 3 bobbleheads, for instance, because they granted some unique modifiers that were cool but not necessary. As opposed to New Vegas's snow globes, which really only gave you money--not very compelling.
Mostly, though, if a game is linear and doesn't let you go back and revisit areas, it's probably best to leave collectibles out altogether, and give gamers other reasons to replay the game.