This game is outstanding. The sheer level of detail and customisation involved will bewilder you at the start and it will take you nearly 20 hours of play before you start to understand.
In terms of gameplay, it is like Oblivion, where you can just do the main quest or try and do a lot of the side-quest (which can be significant as well). Although, it is unlike Oblivion, as you have guns (lots of guns) and the VATS combat system is AWESOME.
If you are expecting lots of cut scenes, you will be dissappointed. Initially, you will have to explore the wasteland and find new places, which can get boring, but once you start interacting, and have lots of quests that you can undertake (and once you can fast travel to a lot of places) the game really takes off.
With the added content, this is a massive game, and very addicitive. If you stick with it, you will not be dissappointed.