It‘s working great for me, I'm using this nice application since a lon time.
It's really easy to switch between languages (I'm writing in Englisch, French and German) - and the biggest advantage for me is: it's one spell checker for all applications, I had to learn it once and it's working for all programs I use in the same way.

It‘s more than a nightmare to try it - I didn't succeed at all to get a demo key.
But I think, I don't need it anymore - I do not understand and accept at all, that an application insists on the first launch to be granted access to the keychain: For what the heck do they want to access password protected features of my system without proper request for it?
This application stuck in an infinite loop requesting access to the keychain - which was not granted, of course!

I'm using this software for years - and it's working fine. I agree partially to archdata's comment, but I'd like to add that it's indeed usable for people without this in-deep knowledge - as long as they stay away from the “More” option...
One big advantage is for me: I'm using it on a notebook travelling around the world with the need to use different scanners of different brands - and using VueScan means: No need to install scanner-specific software or drivers, no hassles with different philosophies on user interfaces - just plug it in and it works for most of the scanners.
I dont't want to mention the support of some scanner-makers for the OS-X platform - VueScan helped me a couple of times to use a scanner being unsupported by the producer!