Ok, not to be a snot, stick in the mud, party pooper, whatever, here, but ...
... to make a long story long ...
Back when I first got the negative scanner I was climbing the learning curve and learning things by trial, error, and experience. I would occasionally post a scanning question on another board. More often than not, the universal one-size-fits-all answer for almost any scanning issue would be a chorus of "get Vuescan". I swear that if I posted that my car would not start or my plumbing was stopped, they would say to get Vuescan.
Vuescan does have a lot of very loyal fans. It must be doing something for somebody, actually, for many.
Well, I did download the demo, set it up, and quite honestly I found it more complicated and more confusing than the stock K-M scanning software.
What I did find out, however, was that my issues were all solved by simply RTFM-ing and playing around a bit with technique, adjustment, and attention to detail when scanning.
So I asked the Vuescan fans if Vuescan would really and truly help out in the following areas:
1. Would Vuescan extract any more information from my negative or slide than the stock software would, when properly operated?
2. Would Vuescan scan negatives or slides faster at the same resolution and such than the stock software?
3. Does Vuescan have any dust/scratch/whatever removal feature that really, truly, actually works. (The stock software has the "auto dust brush" which is about as effective as a placebo on a staph infection!)
I think there was another question, but I forget.
Anyway, the bottom line was no on all questions.
I can see one advantage to Vuescan. It would allow me to use both the negative scanner and the HP flatbed with a single interface. However, since I use them for different purposes in different workflows, I don't see any major benefit, plus one disadvantage of having one more thing to buy, one more thing to learn, and one more thing to upgrade and maintain.
Bottom line, for me, anyway, the stock K-M (and HP) software works fine.
My guess is that Vuescan was written in response to some scanning software that was really ugly and ill-behaved.
My suggestion is that if the stock software works for you, use it. If it does not, try Vuescan, but don't expect it to extract detail where there is none, remove dust, clean out the cat box, etc.
That's my take on it, although I think it's not the most popular.