My personal experience is that the last 10% are hard when:

  - I didn't stick to the specs
  - Added specs while coding
  - Didn't test while coding
  - Didn't fix shitty while coding

Because I think specs are very important I don't believe in too much of agile. I think the waterfall model is very useful but you have to keep the waterfalls very small. Maybe you can call this the agile waterfall model: waterfall -> release -> waterfall -> release

Related: http://en.wikipedia.org/wiki/Ninety-ninety_rule