-Flex requires control. I recommend reading Pat's article, "Lighten Up" which talks more about this. A light touch is highly recommended for flex nibs, otherwise a fine flex will write like a broad. And the nib can be sprung, meaning it will not bend back anymore. The more flexible a nib, the more control is required to prevent the nib from being sprung.
         -Flex requires skill for nice handwriting with line width variation. With an italic nib you can write normally and your letters look more interesting without effort. The shape of the italic tip gives your writing line width variation. Flex nibs allow your letters to look really fancy by varying the pressure when you write. But you will not be able to write with a Spencerian hand without many hours of practice.
         -Flexible nibs are more likely to skip if you use too much pressure.
         -Flexible nibs tend to be wetter than their stiff counterparts. If you don't like a wet nib then flex may not appeal to you.

         Even with those caveats, I strongly recommend trying a flex nib. While you may not have the skill to write Spencerian script, (see bottom of page) flex nibs have a unique soft feel which makes them fun to use. For a first flexible nib, I recommend trying a semi-flex like the Namiki Falcon or any vintage pen that has a mild amount of flex. Also, Pelikan's M1000 is another good candidate, though it is usually more flexible than the Falcon. Fine or extra fine nib are typically good candidates because they allow for greater line width variation. Once you're really brave you may want to try a flexible italic. These are much more position sensitive and more prone to skipping, so you must be careful both with your light touch and the angle of the tip. Whatever you choose, experimentation is always my recommendation.