It looks like a very good game.
Is it worth downloading?is it based on Battletech?

Are you talking about Mechwarrior 4: Mercenaries, or Mechwarrior 4: Vengeance?

Both are really good, but Mercenaries is the one that is free. Remember to download the hotfixes (version c is the latest one, and fixes a serous crashbug late in the campaign). It is based on Battletech.

Mercenaries.do you know a site where I can download ?

Vengeance has really bad acting (which, among other things, ruins the otherwise cool intro), but other than that I quite liked it. Some missions are very tough, but even I had the patience to work through them.

I think Mercenaries was better in every way, though.

I liked Vengeance more (I still have the cds).
Mercenaries I ended playing when it became free. Very good but not as good as Vengeance in my opinion.
I don't think you can buy Vengeance anymore (perhaps on Ebay.)

Mercenaries can be found here (Don't know if still works):

http://www.mektek.net/projects/mw4/

You need the MTX client to download it for free. It can be found here: http://www.mektek.net/ (and also on fileplanet, in case their server is being buggy again)