The Clarisonic Mia is a sonic cleansing tool. It is just a smaller version of the Clarisonic Classic and the Clarisonic Classic Plus and is marketed as a "travel size" Clarisonic. These tools are supposed to do a much better job of getting all the makeup and oils out of your skin than any other method, thereby allowing your products to absorb better.
The Opal infusion is entirely different. You use it to help "infuse" a product after you have cleansed your skin with one of the above. It is a tool that is supposed to help maximize absorption of whatever eye serum you use by "tapping the skin around your eye at 125 times per second." They sell their own anti-aging serum to use with the Opal, but you can use a different product if you are happy with what you use--although I think it has to be a gel.
I own both the Clarisonic Classic and the Opal. I have had the Clarisonic for about 6 or 7 years, and I love it. I use it twice a day. I believe it does a better job of cleansing than anything I have ever used. I also own the Opal. I don't often use it--not because I don't think it works, but because it is just one more thing I have to do before I go to bed and I just don't want to take the time to do it, even though my aesthetician insists that it would make a difference. Maybe someone else on this board can speak to whether or not consistent use of the Opal is worth the time.
I will also add that this company has fantastic customer service. HTH