I have been using Ubuntu for the past couple of years primarily as my browsing OS, so still have Windows on a different drive for gaming and other Windows based things that I still need it for. But really, I cannot find anything to complain about with regards to Ubuntu - runs decently fast on my PC which is ashamedly very very old. Using current version 9.04 and it's very nice. Stable and smooth!
I have not yet experimented with Mint, hear good things and will no doubt give it a try at some point - also if the freedom toasters had them. Mandriva, well I used to use Mandrake all those years ago, but tried Mandriva last year some time - installation was not as sleek as the old days of Mandrake, but installed and on first attempt, trashed my Windows partition which was on the same drive then. After reinstalling Windows and second attempt everything installed fine and then on playing around with it I was just not being wow'd by it. I then tried its 3D desktop option and at some point something broke and the OS was literally crippled - wouldn't boot up. So trashed it and went back to Ubuntu...
Although I personally had a bad experience with Mandriva, doesn't mean to say that it is rubbish. Still very much worth trying and there have been at least one or two versions since my experiences with it.
I'd say give Ubuntu a try as much as I would say give Mint a try at the end of the day...
Philip.