I used to own a dog when I was younger, about 20 years ago we used to have a border collie and she was a great dog but she was hard work.
I would never get another dog as a pet again though simply because they're high maintenance.
Size of dog does make a huge difference - owning a great dane is going to be harder work than owning a Chihuahua, although those yappy little things can be a pain in the arse as well, you can just pick em up and dump em on the treadmill.
Don't get me wrong, I love dogs - but even with my 3 cats, they're easier to manage, mainly because they can let themselves in and out of the house using the cat-flap, so they're out all day.
Only time they come in is for something to eat or to sleep.
Still a pain when me and my wife go on holiday though - my parents have to come over to cat-sit for a fortnight from the other side of the country, otherwise, we'd have to sort out holding them up in a cattery down the road (which costs a fortune)
Don't think I'll be replacing our cats once they're gone though - I'll be done with pets by then I think.