I wouldn't quite put it at "basically free", since it is for only one sector. Multiply that by 4 and add the economy fare, and you're probably around the "standard" I class fare...

So not quite free, but definitely "fair"... QR has made it very clear that it is not trying to fill up it's C cabins to make more space in the back. In fact I'm still sort of surprised they actually offer upgrades for cash.

The question always is, is it worth it for YOU. Depending on schedule, level of anticipated tiredness etc, it might be. Considering DOH-VIE is less than 5 hours, I would't go for it. Same offer for a night flight >7 hours.. Worth a thought.