Ok, what makes it good? What do you do in it? How does it compare to strategy and RPG videogames? Is it free and works in (shudder) Windows Vista?

One of the better free games that I've played. Challenging for me, although I'm a fairly casual strategy gamer.
I have Vista; it worked with no problems.
Beyond that, it's a fairly generic TBS fantasy game with a focus on unit tactics. Lots of additional content available online, too.

It works on Windows, Mac, and Linux. It's pretty good, if you like RTS strategy games.

The good thing about it is that it's quite challenging - the scenarios and campaigns are all user designed and you can download plenty of them for free.

I find it gets a bit samey after a while, but it's definitely worth playing.

For a free game its incredible big, with some long excellent campaigns.
The AI is servicable, but some of the battles go on for ages, and are full of surprises.
 
I recall downloading this when I went away on a week's conference a few years back, and this game kep me occupied every evening instead of porn hunting.

It's perhaps the best thing to happen to TBS' (turn-based strategy games) in years. It gave a framework and a sandbox editor for other people to create their own TBS campaigns and release their ideas unto the world. Not wholly unheard of, as other games have done the same. But critical because now is the age best situated for ease of file sharing and online multiplayer play.

Considering TBS are dying due to the RTS (real time strategy) games overwhelming the strat market, this has been great. It gives the community fans a standardized toy to tinker with, and an easy means of distribution. Given that TBS don't look like they'll come back much from their current niche, which tends to kill any importation from other overseas TBSes, this might be the best thing available for the next few years.

I can't wait to see if someone has emulated some more famous TBS games of the past like Shining Force of Langrisser (Warsong)!

It's pretty cool, but it ain't no Fantasy General .