Blenders are considered convenient when the setup and operation are simple and/or when the product comes with several container options.
For the most part, users consider single-serve blenders to be the most convenient, and generally prefer jars with a capacity of 48oz or under. While blenders this size are not optimal for processing hard food, they make excellent blended drinks. They are also relatively inexpensive, with the majority of personal models costing under $100.
NutriBullet and Oster both rank highly in this category.