I normally just explore the map and do dungeons if it helps

Pros:
start at 55
perma pet (unholy spec)
Death Grip
zombie army
nice DPS and health
veteran players can solo some group content as blood (2-3 people elite quests for example)
Runeforging (free enchantint essentially for your weapons, unfortunately it cannot be in conjunction with actually enchants)

Cons:
gotta have a level 55 character first
people will expect you to tank, so if you do not know how to tank i'd do some research
death grip (in dungeon groups and not tanking hitting that button could get tanks mad at you)
have to put up with being called a deathnoob
need to understand how the rune resource works (how to use the runes themselves and runic power)

They are fun, new, exciting, and pretty pro if you know how to cycle correctly..

I don't like them because I honestly can't play a DK very well and they are kinda confusing to me but I know people who love them.