Yes, I would definitely recommend having some zigbee repeaters. They take some load off the hub and generally traffic just moves much more smoothly through the network with fewer lost messages. Also, the hub has a fixed limit of 32 non-repeating Zigbee devices which can be directly attached to it, and that can throw things off as well.
I just use whatever zigbee pocket socket is the least expensive, but a lot of people like the iris smart plug which is a zigbee pocket socket which also has a separate Z wave radio in it, and so acts as a repeater for both zwave and zigbee.