I've never owned a Cairn Terrier but I had a friend who did. It was a wonderful energetic little dog that just loved to play and be loved on. He was also protective, and would bark at strangers until he was 'introduced'. Good luck with them!
I love my cairn, he's a big dog in a little dog costume. He's now 14 1/2 years old, loves my kids, cats and other dogs. He's the first dog I EVER owned (didn't have dogs growing up), and I could never replace him. He's funny, smart, fast, loves to retrieve and cuddle, plays in the water, and is just a joy. They can be a bit barky, but training helps. Their coats don't mat unless you never ever brush, and dirt and leaves just sort of fall off - they dont' carry stuff around like cockers with long fur on the legs. I highly recommend them to people with kids, I had the dog before either of my kids, and never had a single issue. Terriers are stubborn by nature, but very smart. The trick is to give him a good reason to learn :)
I agree with Leesgirlrn. I currently have a 17 year old and he is absolutely wonderful...my best friend! We take him everywhere with us from sailing, canoeing, hiking to shopping at Petsmart and any other place that allows dogs inside. The love to learn but quickly get bored so don't expect them to do the same thing over and over again. They are great little agility and obedience dogs because they are so intelligent and love to show off. Visit Col. Potters Cairn Terrier Rescue site and you will find that they have a wealth of information on Cairns from temperment to grooming and everything in between. I found it very helpful. I am adopting a rescue Cairn so my 17 year old has a playmate now that I live in a house instead of an apartment and have a fenced in yard. I can't say enough about them they are great little companions and unlike a lot of little dogs, not yappy. Of course that is not to say that they won't bark just to see what they can stir up (like a cat or squirrel)! The one compliment I always get (besides he is so cute!), is that he is so well behaved! Training is a must for any terrier though cause they do tend to be a tad stubborn and independent, but if you make it fun, they are quick little learners! Good luck with your new cairn.