My understanding is that although lots of shelters are doing neuterings as young as 8 weeks, this is a rather recent development and research can't be complete for another several years. It may not affect dogs much at all, or it may eventually be shown to cause things we'd hate to see.

We neutered Eli at 7 months--he has none of the bad habits like humping and marking, but has still grown to full maturity and is quite "male".