I haven't had any K-cup brewed coffee that is even remotely drinkable, compared with a good drip coffeemaker with reasonably good coffee beans. Of course, I burr grind my coffee beans fresh every morning and use bottled or filtered water, not tap water.
Workplace has 'free' Keurig coffee for anyone anytime, and compared with stuff I brew and bring in a thermos, it tastes horrible - bitter and no depth to the flavor at all. (Anytime I forget to toss the thermos in the car, I curse myself for having to drink the work crap.) Most K-cup coffees start with very cheap, low quality beans, and the brewing process runs too fast to get any of the good flavor out of the grounds.
Now, if you add a lot of sugar or flavorings to your coffee, it probably tastes fine, but I enjoy my coffee black and freshly brewed.
I've never tried one of the 're-usable' K-cups to put 'good' and freshly ground coffee into, to compare the output with a standard drip maker or percolator. But my guess is it'll still taste very flat and bitter. Good percolators that filter the water thru the grounds many times over during brewing generally make the best overall coffee. But Americans seem to like 'convenience' over quality and flavor...Keurigs are to coffee what McDonalds is to burgers, IMO.
Another tip: if you are buying a drip coffeemaker, get one with the thermal carafe (I have a Melita), not the glass ones with the heating element to keep the coffee warm. Every drip maker I've tried w/ heated glass carafes the coffee tastes pretty bad within an hour after brewing, because the heater 'boils away' all the good tasting volatile compounds and the coffee starts tasting burnt. A thermal carafe keeps the coffee warm and preserves the flavor for several hours. Alternatively, if you have a good glass carafe maker, just get a stainless vacuum insulated thermos to pour the coffee into right after it's finished - it will retain its good flavor for many hours throughout the day vs. the burnt crap you have on the burner within 45 minutes.