We've got a trailer that we put the children in. You don't notice the weight too much until you go up a hill, or down one, so it's probably a good idea to make sure your brakes are in good order.
It's a good idea to make sure your bike will accept the tow bar fitting though. My bike has quick release wheels so it was a simple job of swapping the wheel skewer for the one that was supplied with the trailer, but to use it on my wife's bike would mean having a different rear wheel built.
I wouldn't fancy towing 60 odd Kgs though rabb_uk!
One thing that might be worthing thinking about, I won't use the trailer on country lanes, I tried it once and nearly came unstuck with a car that came flying round a blind bend behind us. It frightened the life out of me and upset the sprogs.
We now stick to cycle paths and places where we are easily seen. Trailers usually have a flag fitted but they're not hugely visible.
Jana,I think the kennel type enclosure on the top will fold down,they sell that type of pet carrier without the trailer and they fold down,plus you can get smaller ones,the one in the piccy is a big one as my dog is a bit of a chunky monkey!!
Have used one for my dog (bearded collie) for years - got it when he was a pup and he is now 15 - and he loves it. When I got it the only trailers readily available were the ones designed for carrying children so I made a cushion to go in the bottom. He wears a harness that is clipped on to the trailer. I've also clamped 2 rear lights to the back and a couple of amber reflectors to the sides. It can be a bit heavy to tow particularly up hill but I've always thought it was worth the effort to be able to get out and about on the bikes and travel further with him. Generally he gets out and walks up any very steep bits.
If in doubt whether your dog will like it then it may be worth hiring one from a bike shop to try it. Several shops hire them out particularly in holiday places and many have pnes specifically for dogs.
Be prepared for lots of comments and even the occassional photo as you travel along!