You ask a few question. I'll try to answer a couple but the question of whether it is worth it I can't answer that for you.
First (IMHO), Renzetti make the finest vice on the market. Some maybe just as good or not as expensive but they make the product all other use as a benchmark. No gimmick.
Next, rotary tying is a style of tying where you rotate the fly to apply materials instead of rotating the material. the result is that you never have a blind spot on your fly, the application point of the material is right in front of you. Once you get the hang of tying in this style you will find that your speed and neatness increase. if you are just starting out learning to tye rotary is easy, if you have been tying for a long time it is harder to get comfortable rotary tying and you may never completely rotary tye.
Not all rotary is the same Renzetti is "True" rotary, meaning that the fly rotates with the hook shank as the axis. Other rotary vices just spin in a circle to give you a full view but do not allow rotary tying. the renzetti vice is quality made in the USA and has a lifetme warranty.
I hope that helps.
for what its worth I have been tying on a renzetti for nearly 20 years including 10 years of commerical tying where i must have tied thousands of flies from size #26 tricos to 6/0 saltwater baitfish, I love the vice so much I have 2, a traveler and a master.