The value of a teaching method is determined by the individual learner. Too often a new or different teaching technique is espoused as superior to others. I am a fan of manipulatives, having successfully used them with a number of struggling math students of various ages. (I taught math at Sylvan for a while.) What I observed, however, is just the same as what is commonly taught in teaching programs - that different students learn differently.
Some students are abstract and logical thinkers, and grasp numerical concepts rapidly without any help beyond a basic demonstration with paper and pencil. Traditional lecture and practice methods are great for these students, while manipulatives may strike them as silly and a waste of time, because they are too simple and time consuming. For these students, manipulatives have a minimal value and can actually do harm by causing the student to disengage.
For students who do not process numbers and verbal instructions as rapidly, manipulatives hold more value. The hands-on method allows them to actually go through the process of manipulating the numbers (hence the name) in a representational form. Once the process is understood through physical modelling, students must then be led to apply their understanding to the symbolic form that is traditional pencil and paper math. This means that manipulative-based teaching will take longer in the classroom, not to mention the set-up and clean up time involved, but the time is well spent if the students grasp the basic concepts that underly the skills that are to follow.
Because students learn differently, it will be best if you provide for both. Use the manipulatives for those who need (or enjoy using - as enjoyment promotes future learning too) manipulatives. Provide a more traditional lesson for the more logical students. Then remember to provide an extra activity for the more logical students, because their lesson will progress more quickly. Be flexible as well. Some students may have an intuitive grasp of some mathematical concepts, but have difficulty visualizing another. Direct students to work in whichever group is most beneficial for them.