IMO, the larger the budget you have, the more valuable the 99designs model becomes.
I agree going to a trusted professional is always a good idea, budget allowing. The value of 99designs at that level isn't so much to crowdsource your fundamental design as it is to quickly generate multiple views/impressions/takes on a particular theme, either for the benefit of your chosen designer, or to present a list of basic designs to the stakeholders. 99designs becomes a way to get a bunch of cheap "looks" at different designs, that your GA and stakeholders can use to help define what it is they're actually looking for.
I agree going to a trusted professional is always a good idea, budget allowing. The value of 99designs at that level isn't so much to crowdsource your fundamental design as it is to quickly generate multiple views/impressions/takes on a particular theme, either for the benefit of your chosen designer, or to present a list of basic designs to the stakeholders. 99designs becomes a way to get a bunch of cheap "looks" at different designs, that your GA and stakeholders can use to help define what it is they're actually looking for.