I had a partial request from an agent and it recently came back declined...and I'm not sure if that's more demoralizing than simply getting the initial query rejected or less. If the query is rejected, you can tell yourself that you probably just didn't get the brilliance of the novel you slaved away at effectively conveyed in the letter. Now getting the partial request is a huge ego boost, especially if it's a different agent in the agency than the one you addressed the query to, writing back surprisingly fast that he's intrigued by it. But then when the partial does get rejected, you can't hide behind self-delusions as easily. The fault is in the writing itself.
It's not all gloom, of course. Getting the partial is a good indication that the idea isn't ridiculously unmarketable or overused. So that's good. And I have high hopes that one of the other agencies I've queried will have a different opinion.