I get different numbers on my Mac (Scala 2.10.1, Java 1.7.0_17-b02, same Darwin kernel as you), but still not very similar to yours:

```scala
scala> th.pbench { appendWithVector(nums) }
Benchmark (1260 calls in 8.486 s)
  Time:    4.028 ms   95% CI 3.864 ms - 4.193 ms   (n=20)
  Garbage: 1.991 ms   (n=199 sweeps measured)
res0: Seq[Int] = Vector(0, 1, 2, ...
scala> th.pbench { appendWithListBuffer(nums) }
Benchmark (5100 calls in 18.05 s)
  Time:    1.110 ms   95% CI 1.063 ms - 1.157 ms   (n=20)
  Garbage: 2.129 ms   (n=209 sweeps measured)
res1: Seq[Int] = List(0, 1, 2, ...
```