When I first started using jUpgrade it was for the one you install on your Joomla 1.5 site and it'll migrate the data over into a clean 1.6+ install (officially called "jUpgrade"). The method sometimes worked and other times it required some patching of jUpgrade to work. Eventually on one site it was failing so badly and I switched over to "jUpgradePro", which you'd install on a 2.x/3.x site and it will pull the data from the 1.5 site. This worked well enough.
On the last site I tried it kept failing too, so I downloaded the newest version of jUpgradePro, 3.1.0. This version has gone through significant rewrites (I've compared parts of the code with my 3.0.x version) and I have to say should NOT have been taken out of beta in it's current state. There are serious flaws and many features do not function correctly or at all. Whether or not 3.1.0 works for you is going to be a crap-shoot. I only got it working after many hours of debugging and making changes to their code.
You don't have much of a choice if you want a free solution, so cross your fingers and hope jUpgrade or jUpgradePro works for you without trouble. If it doesn't, pay for support or pay for one of the other migration extensions.
I am giving his extension an average rating because of how buggy 3.1.0 (the current version) is and because their support business model is very flawed. You cannot pay a small fee to get support for just jUpgrade. You have to pay a LARGE fee to get a year long subscription to all of their products. It's cost prohibitive for someone who just needs help getting this one extension to work correctly. With how buggy 3.1.0 is, most users are going to be frustrated and have no where to turn.
It works mostly as expected. I like that it uses Joomla's built in mootools instead of trying to include some other JS framework. Most of the meat of the module is in the template file, which is good since most people will want to override the default template to make the module fit their site's design.
It biggest issue with this module is the developer hasn't fixed the fatal bug in the logout form (a missing token) which renders the module useless out of the box. The nature of the bug indicates the developer hasn't tested their module on J!1.6. The fact that they were informed of the bug a month and a half before this review and haven't fixed it yet indicates to me that they are not offering any level of support for this extension.
So, if you're a developer who just wants a starting off point for an inline-popup login/register module than this may get you what you want. If you don't have any technical experience then I'd be wary of the J!1.6 version.