Shimano's pricing model is pretty effective, and the companies that use their stuff have the market figured out nicely.
Most serious bikes come with a minimum of 105, which is perfectly functional.
Folks that want to splash out a bit on the purchase get Ultegra and it is a bit nicer, lighter and shinier.
Dura ace and Di2 is for those who don't care about the price.
All of these groups are good quality, in fact even the lower level groups are quite serviceable.
Bottom line for new bike buyers:
If you are counting nickles, get 105 and spend your upgrade bucks on wheels.
If you have the cash and you want something nice, just get the one you like that doesn't bust your budget.