I have heard people rail on that open source reduces coding time because it prevents you from having to reinvent the wheel. This argument is only true in part, because sometimes it is necessary to optimize the code heavly, which means making certain assumptions about your domain. Sometimes, the open source layer, is on the other extreme and made to many assumptions for your application. Please note that closed source API's are no different in this regard. So in the end keep in mind that sometimes, you might be stuck rewriting, or at least thoroughly revamping something as ordinary as a MP3 decoder.
Open source code is generally very poorly documented, when you buy source code from a vendor it is typically documented better and you will get support for the code, even if you need to change it. They will first see if its possible to do what you want to do without modification of the base API, this is a good thing. However if you find that this is not the case they should help you muck their code base into another hemisphere.