This is an effect that I'm kind of so-so on.  On one hand, its a cool way to visually unify an entire run and if you have a particularly packed bookshelf you can find a paricular series at a glance.  Handy.  On the other hand, it can seem a little gimmicky.

That being said, I think there are some really cool examples out there.  One that sticks out to me in particular is the spine art for the 42 book Dragon Ball series:



Like anything, in some cases it works and in some it doesn't and it really comes down to a case by case basis and also the subject matter of the work.  For a multi-volume collection of Silver/Bronze Age Amazing Spider-Man I think having all the spines create a giant "Amazing Spider-Man" logo would be neat despite the tackiness of it (because Spider-Man can get away with tackiness!)