Of course, it's not a black and white question, and every dog comes with its own problems and quirks. However, some insight into the life of a big dog owner may help sway you in your decision.
As a child, I owned medium-sized dogs like Springer Spaniels and I loved every second of it. They were playful, big enough to cuddle, and bouncing with energy. As I grew up, I decided I would take the plunge of getting a big dog. A Dobermann no less. I have never looked back. (But that doesn't mean it doesn't come with its disadvantages!)
A Good Idea?
My favourite thing about my big dog is hands down the human-sized hugs I get. There's nothing like getting a huge warm welcome everyday, and it's a surprisingly gentle welcome. This leads me to the next positive: big dogs are well known for their calm temperaments (evil film hounds aside). This is because many big dogs have nothing to prove when it comes to dominance in society whereas smaller dogs tend to be more jumpy and active to intimidate bigger dogs.
Big dogs like Dobermanns and German Shepherds are renowned for being good guard dogs after earning their scary reputation in films like The Omen. It's as much a positive as a negative. But I can't help but feel secure knowing I'm never home alone. (Although, my dog is a case of all bark and no bite when it comes to visitors.)
Like I said before, big dogs usually are very calm and patient, which means they are very good with children. They are less likely to be agitated when manhandled or being played with roughly. However, it goes without saying, kids should be taught that isn't the way to treat a dog as every dog will have its own limits. Another benefit of their patience is that it usually means they're much easier to train at an early stage. And, with their reputation, it's probably better to train them.
A Bad Idea?
As I have hinted at several times, the heavy usage of big dogs as vicious sidekicks in movies has made many people very apprehensive of big dogs. So, be prepared for people crossing the road or shielding their children as you walk past them, they simply don't know how much of a big softie your giant really is.
Some big breeds require lots of walking, these are usually hunting breeds as guard dogs are content to sit all day. Walking is essential for owners who want to keep a big dog in a small space such as an apartment. To keep a big dog it's ideal to give it lots of space and room to roam around BUT that doesn't mean they can't be kept in smaller homes like I said above.
Expect it to be pricey. These dogs cost more to feed, cost more to be treated at the vets, and cost more to be entertained. Toys may be shredded in seconds so don't plan on them cuddling with soft teddy bears. (However, I recommend deer antlers and the Kong collection to stop them in their tracks.)
Finally, as a general rule, the bigger the dog is, the shorter it's lifespan will be. The average big dog will live around 10-14 years (it will change depending on the breed) so that may be something to consider if it will be a family pet.
It needs to be mentioned that there are some bad behavioral traits in some large breeds but this is down to the individual dog.
There are positive and negatives to all dogs no matter what size they are. It all depends on whether the potential owner has the time and effort to give the dog an enjoyable life.
If you do decide on getting a big dog I would also recommend looking into rehoming a dog. There are lots of large breeds in dog shelters as these breeds tend to have larger litters and some cannot be homed.
(All of these points are generalisations as every dog has a different personality and background. Therefore it is just as important to consider the individual dog that you may get.)