There are many types on the market, those that plug in to a wall socket and record little more than the average, max & min, and possibly transients. Unfortunately, they are also renowned for driving a power quality investigation straight up the wrong street. Believe me, it's not a nice feeling going up the garden path only to have the front door slammed in your face.
I won't mention names but I saw one such device that was intended to record the number of 'serious' fluctuations. In one day alone over 1000 'triggers' had been counted. This would most likely be caused by a loose connection or similar fault? Wrong! It shared the same circuit as two panel style oil-filled heaters whose temperature controls were similar to those used on electric cookers; the type that uses a small bi-metal strip wrapped in a heater wire.
These are no good for faults beyond "my lights are very dim at night" where all that is required is the voltage at night. If you have made wild claims that "our company will have an investigation under way in 7 days" then these will go a long way to saving face - the results, however, may bring you no closer to the conclusion.