Generally speaking, a device designed for flower will come in two varieties: convection and conduction.
The conduction style flower vaporizers put your herbs in an oven-like setting and heat the walls. This warms your herbs by a transfer of heat through the oven walls. It leads to the herbs closer to the walls getting warm/vaped before the herbs near the center. One more thing to note about the conduction vaporizers is the time it will take for the oven to heat and your herbs to heat after that. Once it is all heated up, you must finish the whole chamber as the herbs will continue to cook whether or not you are taking draws.
The convection style flower vaporizers put your herbs in a chamber with a heater placed near-by. As you draw from the device, you pull air over the heater, which is warmed to temp and then that hot air goes through your herbs. This offers a more even vaping experience. Another benefit is that the only time spent waiting is for the heater to come to temp. You warm your herbs as you draw, so there is less wait time and this also offers you the chance to take a draw or two at a time rather than finishing the whole herb chamber.
I feel the convenience of packing your flower of choice right into your vaporizer combined with the money saved (herb is extracted much more efficiently and herb costs less than oils) and the minimized waste (no cartridges, etc.) makes a dry herb vape pen a no brainer!
My preference is the Grasshopper Vaporizer. For more information, checkout ghvape.com.