I'd say it's totally useful, simply because it's O.N.U official language and Russians don't speak English in general, it's not like you go to Germany and you can survive your entire life wtih English everywhere. So if you speak it, you're a special one! 

Last but not least, it's a great language for brain exercising! It's extremely difficult, not only its monstreous grammar, but also the alphabet, so once you start learning it, it's like mathematics, it'll keep your brain active and will avoid some future diseases like dementia! 

You are totally wrong! Arabic as Russian is a difficult language for most of the people in the world! It's totally useful and companies would pay someone here in Brazil very high salaries if they could speak Arabic in order to do business in Arabic speaking countries! 

Once I worked as an interpreter at an international event in my state and there were film directors from Europe and China! The ones from Europe, like Czech Republic, Germany and Switzerland had English speaking interpreters because these people can speak English and it is easy to find interpreters for English but the poor film director from China was horrible at English, because Chinese people also suck at foreign languages like us Brazilians and Russians, but unfortunately the event didn't find ANY Brazilian who could speak Mandarin, so they had to hire and pay 7 times more a Chinese who could speak Portuguese. 

So, if you speak Russian, you'll be unique! Trust me! 

It depends on your definition of 'useful'! For me, language learning is about building bridges between cultures, and there's no better time to build bridges than now, if you look at international politics.

Artyom, in Fodeil's case - maybe. But as the first lanuage it is still useful:-)

Fodeil, it isn't useless the way you've defined it. For reading scietific literature proper you need just the basic knowlege of a language. Vast majority of journals is published in English. It is easy to read it as long as you either understand the field or eager to understand the field. As to textbooks...
Whatever exist in other languages would usually exist in English. So you can read everything you need in English and I really hope you do read!

There are some exceptions, mostly in humanities. If you studied phylosophy - it is much different in different countries. If you studied Arabic dialectology - welcome to Germany. Etc.

But: you can still become a physicist, a mathematician, a biologist, an engineer here without any decent knowlege of English (though later you will need to read journal articles in english). We are famously not good at speaking English, but we still export people mentioned above (no wonder. science isn't well paid here).
Exception being some modern fields and subfields, where literature is scarce.

Now demand... Rare languages (I'm not sure if Russian is exactly rare) assume a different employment strategy. Some Arabic companies can lead business in pretty exotic places and vice versa. Don't even think that Burmese is 'useless' unless there doesn't exist a single one Arabic/Algerian company working with them:)
But Russia used to collaborate a lot with Arabic world...

Having this said, I wouldn't advice you Russian, because there are more attractive options around:)
Apart from other European languages.... Do you know many Algerians learning, for example, Chinese? Korean? But given the size of these economies... there MUST be some demand.

It depends on your goals. I'd say it's completely (can't stress that word enough) useless.