They are worth getting if you enjoyed the base game, the reviewers scored them low from an impartial standpoint. Fallout fans will love these add ons. It is true that you can't travel back to the Sierra Madre after completion of Dead Money, so you may want to use the Vault to be able to find everything in the first go; the bobblehead Snow Globe is extremely hard to find. I even missed it in 2/3 playthroughs.
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Fixed it...

With respect to the OP, I loved all of the DLCs. My synopsis of the DLCs without spoiling them are: HH is easiest of the DLCs, looks good, is a good place to hunt and gather for crafting, and arguably has the best gear in comparison to the other DLCs; OWB has the best humor and the best "home base" that can be used after the DLC quest is over giving it the best "long term" value; DM is easily the most challenging of all the DLCs, so if you are looking for one that's the one to buy. IMHO if you are a fan of FONV, you should get all 3...