Should I Buy One?
If you’re doing a lot of macro photography in the field, or are shooting a lot of relatively close-up portraits or fashion images, then a ring flash can be a good investment. They don’t come cheap, however, so make sure you really need one before buying unless you have money to burn.
One way to see how much you’d use a ring flash, or if you even like the lighting effect it gives your particular photographic style, is to try before buying. If you have a photographic rental shop nearby, you can rent one, or you can also get an attachment for a normal shoe-mounted flash that (sort of) turns it into a ring flash by diffusing and bending the light around the lens. You can also try this DIY tutorial to create your own budget ring flash using a standard external flash unit and a couple of plastic bowls.
Craft and hobby stores also sell a kind of work lamp with a circular light surrounding a magnifying glass. If you take the glass out, you can use the lamp as a makeshift ring light, though obviously it's not portable and would only be useful in a home studio where you can plug it into a wall socket. You may also find that a desk lamp doesn't produce enough light for your purposes. Make sure to check what kind of bulb is used in the lamp and compensate with filters or camera settings, or remember to adjust the color balance in your photo editing software later as different bulb types will produce different color casts in the image.
Once you've tried a ring flash, either by renting a real one or constructing a makeshift unit, you'll soon discover if you find yourself reaching for it all the time. Then you’ll know investing in one might be worth the money. If the makeshift one works well enough, though, you could spend the money on something else.