Is the addon worth it? Love the original and play it often.
If you enjoyed DW vanilla, you'll find RotS to be well worth the (rather small) price. The improvements to the UI and the depth added from the mechanics changes (missiles, fighters, tech tree, etc) to be quite dramatically increase the replayability of the game.
As a caveat, however - grab the latest patch as well as the one previous to it. Depending on your playing style, you may find that the latest patch creates more (or more significant) bugs than it fixes. I had this with 1.502, which was problematic for me. I found that 1.501 was more playable, till 1.504 became available.
And finally, remember that it's a 4X game - winning isn't the entire goal. It's HOW you win that's equally important. If you find things too easy - try another race, try different tactics. Found it too easy with the war-mongering Boskara and their torpedoes-from-hell? Play as the Teekan or Securan, and you'll find your Boskara derived tactics... well, I don't want to spoil the surprise for you.