Historically, cheap oil is good for the economy.
I think it depends on the economy. Countries that are heavy dependent on oil production might disagree. When a big chunk of the GDP goes down, a lot of sectors are affected. Big companies survive, but they also need to cut their losses and you can see how many people lost their jobs in the past year and will continue to lose their job in the following. Small and even medium sized companies are even in a worse situation, especially service companies who might be left without any contracts whatsoever. So, not everyone is jumping for joy with more money to spend. You really have to look at all the ramifications of the oil crisis, they're not just limited to the oil industry itself.
Now, on the topic of investing into oil, I think it all depends on how long you're willing to wait.
I mean, there's a lot of speculation involved, my personal opinion is that the price of oil will continue to slightly decrease for the following 6 months, but it should be back to over 100$/barrel in less than 5 years. That should make an investment in crude oil future ETFs (BNO maybe) quite profitable for that time-span. Do note that I can't predict the future, so my guess is as good as any.
For long term, big oil companies are the way to go, as long as you keep it diversified (VDE), but I doubt they'll bounce back as fast as the oil price. There will be a lot of inertia, getting operations started again, bringing people back into the companies, these will be considerable expenses on their side, also taking into account that drilling costs will go up. Then, there might be other factors as well on the long term. After the Global Climate meeting, who knows what taxes might be enforced on oil producers and their products. Still, big oil companies will recover and go for big profits, and as it was mentioned, you can always count on dividends in the mean time (also, for non-US citizens investing in US stocks, don't forget the dividend tax).
