95% of the time they detract from the experience rather than add to it. IME. Especially when they come on an album when it is first released instead of on a reissue. For every bonus track that is awesome (tons of ones on the Judas Priest reissues are fucking awesome) there are ten that are lackluster live versions or demos I have no interest in hearing. Covers are a mixed bag, if you do a cover you should own it hard enough that it can appear in the continuity of the original album. A la,
[VIDEO]http://www.youtube.com/watch?v=Pb30GOdjlE8]Arsis - Roses On White Lace - YouTube[/VIDEO]
I mean, if I am buying a print of Da Vinci's Mona Lisa I honestly have no interest in the doodle he drew on a napkin before doing the proper painting.
In some cases they really work. Good examples for me are Scar Symmetry's Deviate From The Form (my favourite track on Pitch Black Progress) or Death's Painkiller. However in other cases they are bloody awful. The last couple of ones from the past two Evergrey albums were just nowhere near as good as the rest of the album.
I think if the album is a concept album, then they tend to really fuck it up. However on something that is more just separate songs without a completely unifying theme they can work without sounding out of place.