Although the two parent breeds look very different and come from regions far from each other, most Cainroodle information sources state that they have many similarities that seem to mesh well. These dogs can be shy if there is a lot of Poodle inherited — especially if it's not a Standard Poodle used. Terriers can be fierce despite their size, and Poodles long ago were used as dogs of war.
The Cairn Terrier Poodle mix is without a doubt a sharp-minded dog. They come from parents breeds who are accustomed to learning many tasks and being adept in agility contests in dog shows. They may not be the best as guide dogs, but they are suited to lots of other types of work.
Depending on how much of which parent is dominant in your Cairnoodle, yours may be very hyper or somewhat reserved. In either case, however, they will need a lot of attention, and they won't like being left alone for very long. They are loyal, which is the upside of constantly wanting you around.
Terriers are known to be somewhat hyper, and Poodles can be a bit more reserved. It's best you have the time to walk a Cairnoodle a couple of times a day for about an hour to make sure he gets enough exercise. Of course, he'll appreciate playtime and even setting up a small obstacle and agility course in the yard!