A tough question to answer. I just started playing two days ago. To me, it "feels" more like Fallout than Oblivion. Besides the obvious wasteland setting, the Pip Boy and S.P.E.C.I.A.L. system are familiar aspects to its predecessors. The real-time combat and camera angle system much more akin to Oblivion, though the optional V.A.T.S. targeting system does use some of the better aspects of the old Fallout system. The dialog system seems to combine the best of both games using skill-based and perk-based dialog options combined with full voice-overs. The jibber-jabber dialog of NPCs in the background seems to have a better repertoire than I remember Oblivion having.
So far I'd say Fallout 3 has its own flavor distinct from F1/F2 and Oblivion. If you liked the earlier Fallouts and Oblivion, I think it's safe to assume you'll enjoy the game play of Fallout 3.