My opinion is that the Fallout 3 DLCs are pretty poor all round, but are required for many mods that make the game a better experience. I've also read (not yet tested, waiting for the game to go on sale) that the GOG GOTY version is more stable than the retail or Steam releases.

You already have the best DLCs. Of the others, The Pitt is OK but the other two are very weak. If all you want to do is play them, I'd say the purchase isn't worth it. But as I said, it might be for other reasons.

The DLCs you mentioned have the same problem as the main game: the story is almost non-existent, and what little story that is there isn't very good.

Point Lookout was the best DLC IMO (except I never played Broken Steel so that could be better, but I've completed all the other DLCs).

My opinion about Mothership Zeta: I enjoyed the alien rifle and the environment as a whole. IMO they did a great job of simulating the inside of an alien spaceship. However, that's the only good thing I can say about it. The characters have no depth. And one of the plot lines they introduce in that DLC becomes completely abandoned, as if the game had never brought it up. That is very unsatisfying and stuck out like a sore thumb for me. And the gameplay gets extremely repetitive after 20 minutes; you are just doing the same thing over and over again... except for 5 minutes at the finale.

The Pitt was alright, but it's a really cliche story (to even call it a "story" is being generous) that's been done in countless RPG games before.

Operation Anchorage has a few cool mountain vistas, and a cool moment or two when enemies assassins come at you in Chinese stealth suits. Other than that, that DLC is pretty bad. It tries to let you choose NPC companions to form your elite forces squad, but except for the specifically scripted one who is voice-acted, the companions' AI is totally incompetent and game-breaking, i.e. they will not enter the building in which 90% of a mission takes place.

Is that stuff worth the purchase for $19.99 USD? No, IMO.

Have you played Fallout: New Vegas? If you have not, then I highly recommend to use your money to buy that. That game is well-worth way more than $19.99 USD. It is still vastly superior to 99%+ of modern AAA titles that cost $50-60 USD and one of the best RPG games ever made (except that most of its DLCs kind of suck, but the main game is fantastic).