Depends where you go. Many GPR programs affiliated with Oral Surgery can be great experiences, because you might even spend "official rotations" on Oral Surgery where your duties and expectations for that rotation period are the same as those of the 1st year resident.
I believe its a greater bennefit to have them as part of your program, because you will have the opportunity to be exposed to a greater # of cases. Some people say there is a lot of annemosity between the OMFS and GPR residents, but I think that is mostly a "personal issue." If you are a good GPR resident, do what is expected of you, and show some initiative and a strong work ethic, then you will gain alliances from the OMFS residents and OMFS attendings.