The expansion packs are alot of fun, but not worth purchasing until you get at least 30 hours out of the game, by then you should have finished the main quest (SAVE BEFORE THAT >_>) and done a fair chunk of levelling and side-quests.

(Trust me, if you like it, you'll get way more than 30 hours out of F3)

Plus, getting it for PC, you can make you own vaults, and make your own everything with the provided tools.