I have two, Seiko the Watch Dog who is just over three, and Paws our clown puppy who is just turning two.
Previously I've always had German Shepherds, Danes are certainly entirely different dogs.
Danes must be house dogs, and are very strongly people oriented.
Danes are by heritage prey dogs, having been breed primarily to hunt and kill wild boar. As such ownership does carry some larger responsibilities.
Danes are relatively low metabolism dogs, Me and my girls walk 5 to 10 km a day, (depending on if I'm working or on days off), and that is more or less enough for them.
I do obedience with the girls, they are not as a breed nearly so eager in this as the Shepherds I used to raise, but they do quite a'right. Both have their Canine Good Citizen status.
Danes are particularly sensitive to quality diet, and I strongly recommend Raw.
Do not feed your Dane puppy, puppy food.
Purchase only from a quality breeder and be prepared for it to take in excess of a year to get on a good breeders puppy list, unless you have references, Dane breeders, good ones, tend to be very particular about the placement of their dogs.
A little mentioned thing about Danes has to do with the colours, in most dogs with different coat colours, the dog is basically the same in any colour, not so with Danes, the different colours are almost like sub-breeds.
The cliche lazy gentle giant is the Fawn and Brindle dogs.
The consummate clown is the Harl lines
Blacks are closest to their original temperament lines and make the best guard dogs, but require more competent handling.
I have a Black, and a Harl breed Mantle. They are night and day apart. Seiko always serious, always on the alert, a very good obedience student, Paws, on the other hand is never serious about anything, and is a bit indifferent in the obedience ring, especially if there is anything interesting happening outside the ring, which means basically anything at all.
These dogs do have a prey drive, it is tamer in the Fawns and most pronounced in the Blacks, so handling requires your recognition of this and an understanding of handling prey breed dogs. Always remember that once a prey dog is on prey, recall becomes a remote possibility, so never off-leash in a place that will test this.
I could go on for pages, I love these dogs, I'll never own another breed now, but I do recognize that these dogs are not for the beginner, and even for an experienced handler like yourself, it is important to realize that you are upping the ante switching to a prey breed.
One last thing, it was said to me by my breeder when I got my first Dane, it didn't make sense, and I certainly didn't accept it... However..."No body has just one Dane."
I've recently bought a bigger house, 'cause I really only did have room for one Dane... *L*.
Danes are by the way, mastiffs, German Mastiffs in fact. Only the English Mastiff uses the mastiff name as it's primary, but if you are interested in mastiffs there is a French Mastiff, and African Mastiff, and Italian Mastiff, an Argentine Mastiff,(a very recent breed this one), a Tibetan Mastiff, a Bull Mastiff.
Of course I think Danes are the very best and most beautiful of them all, wait until you see your Dane at a full run, 'tis a thing of beauty beyond words.