In reply to James Malloch:
Worth considering: a longer rope means significantly more weight to carry on approaches and way more length to take in at belays. I have a 70m rope, which is massive overkill for most things I climb, even using it folded in half (it's triple-rated) as doubles.
The crucial thing, obviously, is what you're planning on climbing: are you going to need a 100m rope? If so, it's obviously not only worth it, it's essential.
If not, though, there are actually disadvantages to having more rope than you need.