Every guy and girl should have one!
Coming from the Philippines, I often see people of both genders toting around umbrellas. However, when I go outside the Phil, those numbers decrease- first the males, then the females. I find this strange, considering the sun rises and sets everyday nearly everywhere inhabited. So if you're someone who walks underneath the sun, and you don't wear clothes that cover 90% of your body, you should have one.
So how do you choose your umbrella? The most important thing is to choose the right umbrella for the weather in which you live (or have an umbrella for every occassion):
If the sun is shining, then you should get an umbrella with silver coating on the inside. No ifs, no buts. Open the umbrella under a light; the less light that passes through, the better. I prefer using darker colors since less light passes through, and less light means less UV rays and headaches.
Btw, it is still extremely important to use a sunscreen/sunblock as light reflects off everywhere!
If its pouring, make sure that the umbrella you use is waterproof! No lace or canvass ones, no matter how pretty they are. Also, no wooden handles or pieces, you might damage them from the constant dampness.
Make sure that your umbrella is reversible. Ask your sales lady if it is, and test it out yourself afterwards: Assume a fencing position, and make quick forward and backward movements of your arm until the umbrella reverses its shape, then do the same to bring it back to its usual shape.
The next most important condition to consider is convenience. Do you want to hold a full size umbrella that can double as a weapon/cane? Or would you prefer a fold-able one you can tote in your bag? Full size umbrellas are generally sturdier, but prone to being forgotten (as you leave them outside/in umbrella stands). Also, if you would like an automatic umbrella, know that your umbrella probably won't be reversible, but could save precious seconds in a downpour.
Less important are size and style.
How many people will be sheltering under the shade of the umbrella? For me, an umbrella that can shelter one person is enough, as everyone I know has one at all times. If you don't have one, your loss. However, I realize that some people prefer bigger ones to shade (potential) loved ones, get less wet (especially when they have a lot of stuff), etc.
As for variety, one has many umbrellas to choose from. I used to have this lovely wood and lace umbrella. Not very effective for rainy days (and for sunny days), but it looked quite nice. You can use your umbrella to express your sense of style! As for color, while black is good because it goes with nearly everything, I prefer more vibrant ones.
My Grosser Schatten foldable umbrella (P390 or less than $8)
Some light still passes through, but not much :)
My umbrella, partially reversed!