Is it worth buying a Canon 50mm f/1.8? What is the lens good at? I read a review that it is good at doing Bokeh. I tried using my Canon 18-200mm to do a Bokeh effect, but it wasn't that good.
I would almost say: "The Canon 50mm f1.8 is ALWAYS worth the buy!!".
It's the cheapest lens you can buy, but what fun can you have with this little piece of plastic. Don't let the plasticness fool you. Of course the image quality isn't as good as the f1.2, but if it comes to value for money, this lens is unbeatable.
The Canon 50mm F1/2 is good at taking good-looking pictures at a normal focal length (for full-frame users), a shallow depth-of-field and short exposure times.
Those characteristics come from the fact it's a 50mm f/1.2 prime lens.
It's also an L-grade canon lens, which means it's professional-grade lens, meant for far-above-average quality, reliability and toughness. From what I've heard of it, it delivers on all accounts.
Do you have to buy this lens in order to make nice pictures ? Hell, no.
Start from the pictures you'd like to take, and try to find out what kind of lenses people use to take these kind of pictures.