Rotary tattoo machines probably come in more shapes and sizes than any other type of tattoo machine. This is because some attempt to be a hybrid between the traditional coil and others intend on being a fully separate system. They use a completely electric system with a motor that transfers the energy to driving the needle bar up and down rather than magnetic coils or air. Rotary tattoo machines are often very light and efficient since there are no metal or rubber parts that wear out but they still use the same voltage as a standard coil machine. Similar to the pneuma style machine, rotary tattoo machines are also very quiet, without the traditional tattoo hum given by the coil system.
There are several advantages of rotary tattoo machines over traditional coil tattoo machines. As mentioned there are less parts that will need replacing or servicing. Most coil machines need tightening, filing and maintenance once a month or so and there are many artists who do not know enough about machine mechanics to have the skills to maintain their machines properly. This can negatively effect the quality of work output and in this case a rotary tattoo machine is ideal. Rotary tattoo machines are also generally much lighter since the frames are ceramic or plastic. Coil machines use iron cores and sometimes iron frames leading to a piece of equipment that can weigh between 1-2 lbs. Some use a completely enclosed system that can be wiped down with disinfectant after use as well as having needle cartridges instead of separate tubes and needle bars.
Though this sounds great there are some disadvantages to rotary tattoo machines. If you have been used to coil machines the swap can be difficult, they are lighter and there is no cut back from the spring. That being said your needle depth is set - there is no give and that is exactly how much goes in; so it is easy to chew up the skin and make a mess if you're unsure or risk having it all fall out if not set deep enough. Really if you want an adjustable machine these are the complete opposite. Professional rotary tattoo machines aren't cheap, and though that can be said of any professional quality machine these average upwards of 2-3 times the price. The needle cartridges are also quite expensive; unlike the grip and needle system where the grip is autoclavable and the repeat costs are only needles the cartridges are often 5 times as expensive even if you do have to factor in less the cost of grip.
Rotary tattoo machines have become quite popular recently with many big name artists promoting them and using them. This does not make them necessarily better than any other type as each artist will have their own needs and preference in a machine. The fact that there also also multiple styles within the rotary type mean that there is a lot to be considered when using and buying one.