It's better for classes with 1d6 than those with 1d12:
Con14 (1 -> 2) = 1/6 1/8 1/10 1/12
Con16 (1,2 -> 3) = 3/6 3/8 3/10 3/12
Con18 (1,2,3 -> 4) = 6/6 6/8 6/10 6/12
Con20 (1,2,3,4 -> 5) = 10/6 10/8 10/10 10/12
For a Con18 wizard, it's worth +1 hp per HD. You might as well get Con20 with all...