Whenever I DM I disallow Splatbooks completely, because, IMO, most of the material is seriously broken. Two of my players whine at this judgement, but I usually remain strict on these terms. I might on an occasion allow them to use Non-Corebook material, but only after I've looked into it. The thing that pisses me off most is when players use Non-Corebook material without asking for my permission. I'll just have to be more strict in the future.