The problems that come in are far to great for this to be a viable method of upgrading. The component will install into a subdirectory, and use the same DB as your original install (doesnt overwrite anything) which is great.
It just seems to be far from complete. I watched as it migrated my components, modules, content, etc only to find that it would not be present once it was finished. I had to install the new K2, which is fine, and once I did, all of my articles showed back up. Only problem is, NONE of the access levels transfer, so you have to manually open and close each menu item, article, category, k2 it, k2 category, etc in order to get the items to publish again. If you don't do this, you'll get hit with 403/404 errors anytime you attempt to access content. It does not migrate any of the component settings that you had, so all categories and items that inherit parent properties will have to be set again. Not a big deal, but if you're going to tell me you're importing my components, make sure they're actually migrating.
I was very, very excited about this component, only to be let down completely. I could have completely migrated to a new 2.5 install by just copying over my articles and manually installing components.
This component has quite a bit of potential, but is no where near ready to be considered the official migration solution, which it seems to be marketed as pretty much anytime you search google for migration options from 1.5 to 2.5
If you are looking for a quick migration solution, and especially if you're not familiar with phpmyadmin (for some issues) do not bother trying this component, as you will end up frustrated that you're site isnt working.
Your best bet is to perform a clean install, migrate your articles yourself and reinstall your components. It may seem overwhelming, but you'll end up doing the exact same thing if you use jUpgrade.
I would like to note though, had I not had ANY third party components installed, and was using j1.5 with only it's native functions, I believe jUpgrade would have worked perfectly.
My problem is the support. I sent a support request over a week ago to fix my categories (scattered all over the page) and the developer claimed he had an idea to fix it and would get back to me. I sent a reply saying thanks and to let me know, and have since heard nothing from them. It is a small issue, but if you say you're going to do something, then it should be done. I was a bit weary of this component based on some prior bad support reviews, but I liked it a lot, so went ahead and purchased.
The component is awesome, the support is non-existant as far as I have experienced.
Something tells me I will see a comment to this review before I actually receive some support.
Anyway, good component, just dont expect much help.