Oven liner? A good idea? Or not?
dmd's right; you have to check either your manual, or if the point is not discussed, call the manufacturer.
One thing you can't do is use them in ovens with glass enclosed bottom elements. At least not mine (Dacor). And just for the reason dmd mentioned; they can fuse to the oven bottom.
I am fairly certain that I remember an article about them in Cooks Illustrated some years ago, and that it didn;t approve of them. I don't recall why. I have also read that you are not supposed to use aluminum foil to line the bottom of the oven and also don't remember the reasoning, but at the time I read that I had been doing so, without ill effect, for several years.
It depends on the oven, but some are A-OK for liners, and on others, liners or especially foil can fuse to the oven bottom. My poor mother learned that one the hard way on a brand-new oven she laid foil over the bottom of, as she'd always done with her previous one when she was baking something prone to bubbling over. Today, she has a large silver/black blotch where it melted.
That's why the manuals on some ranges warn you not to use foil when you're convection baking -- the fan can blow the foil off and onto the floor of the oven.
Oven liners are great, particularly if you are renting and/or don't have a self-cleaning oven. The ability to either remove the liner to clean (instead of losing the oven for hours for the self cleaner) or recycle is a great help, particularly when making a lot of food, ie during the holidays. If you're roasting a bird or baking a pie which spills, it's much easier to clean the liner than scrub the bottom of the oven.
I've never had a problem with heat distribution being affected, and haven't found brand to matter (the ones at the dollar store work well enough).