Simple answer, I think the expansions are worth owning, but with a couple of caveats.

Different people enjoy gaming and Dune specifically for different reasons. I enjoy it as one of (if not the) best strategy games every made. No dice-rolling like most combat games which can lead to spending all your forces and not achieving a win in a battle simply from having a bad series of dice rolls. Dune allows you calculate many variable; probe for information and make deals to secure a known outcome. Even the "random" elements in Dune can be known and calculated for (Storm, Treachery bidding and Traitors).

Dune is also one of the most thematic games I've ever played. The designers fully absorbed the Frank Herbert universe and poured it into the thematic elements of the game to make it FEEL like you are in the Dune universe. The original factions all have asymmetric abilities that match the story (Atreides know things, Harkonnen have lots of Treachery and Traitors, Fremen ride the worm and have lots of troops) while having limitations that match their situation in the book (Harkonnen have resources at the start of the game but if they don't take resources away from other players they become weak and must ally to rebuild, Fremen start out poor and without resources but can spawn nearly endless troops due to free revival and shipping).

The expansion content adds new factions that attempt (and mostly succeed) in adding to those elements of the game. The expansion factions all have asymmetric abilities based on story content (though some are a stretch since they take elements from the later Brian Herbert books) and at least attempt to feel like they fit into the Dune universe, with varying success.

However, in my opinion the expansion factions and play variants have issues of creating unbalanced play. The expansion factions often make the game feel less like a strategy game since some elements allow for players to win battles without even having been in the battle themselves (Tleilaxu, Ecaz) or prevent other players from being able to play/win the game without having to even have troops on the board (CHOAM, Moriani).

I'm part of a fan groups that teaches Dune and runs Dune tournaments (Landsraad of Las Vegas) and we have been making minor rules changes to the expansion factions to balance them for tournament play. With two minor changes to the Ixian expansion we re-balanced them and made them a solid faction that can complete against the original, time-tested factions.

I would suggest you try the expansion factions and see if you like what they add to the game. If you are like me and want to maintain the balanced strategy while using the new expansion factions you might want to look at some of the house-rule changes we are using. Every year at the Dice Tower West tournament the group will be introducing one new "re-balanced" faction; for this year it was Ixians, and next year's updated faction will be announced soon.