General rule of thumb...
Good: A, B, C, D, E, F, G, H, I, L, M, N, O, P, R, S, T
So-so: J, K, U, V, W
Not so good: Q, X, Y, Z
Although due to Queensland, "Q" may be slightly better in the .au space.
Values are about the same as a length of string.
check out this site -
http://www.3character.com/
good info on these domains, yet more focused to overseas TLD's still decent info.