Do it! They are gorgeous, loving, gentle dogs!
Yes, they do tend to have a shorter life span than other breeds but I don't think that should be a deterrent, as the time you do get with them is so wonderful :)
They are also prone to a number of health problems due to their size:
Bloat - very dangerous but the risk can be reduced by feeding them from a raised bowl and feeding small meals throughout the day (3/4 times a day).
Arthritis and Hip Dysplasia - Bone and joint problems in general are more common but again risk can be reduced by not allowing them to jump and run too much, especially when young and the bones are still growing.
Unfortunately problems like cardiomyopathy (enlarged heart) there isn't really much you can do about it, they will get it or not whatever you do. One thing I would really recommend is insurance with lifetime cover, ie, if they develop a condition like arthritis which will need long term care you are covered for that condition for the life of the dog, not just the first year.
Dane vet care tends to be pretty expensive due to their size but if you have a good insurance company this should take care of most of it.
They don't need that much exercise, don't really shed, are usually good with kids and other dogs, curious rather than aggressive but obviously ever dog is different!
Totally recommend them, just do a bit of research :D