The big 0.8mm ball feels much better to write with, and the ink is really great. While you don't have the same variety in ink as fountain pens, the black is really dark (& waterproof), and Uni-Ball has some nice BLX colors - the dark blue is also great. But the rollerballs are less sensitive to any imperfections in the paper, will write at any angle and rotation, never leak (so I don't need to constantly pay attention to what angle it is in in my pocket), don't get ink all over your fingers, and don't need special handling to take into airplanes. Just much more practical overall, and more of just a tool that works rather than an art piece that needs strict care (analogy).
Using rollerballs is more expensive over time; a bit less so if you buy refills instead of a new pen every time. But it's worth the convenience. And while I do keep them close since they're expensive, they're cheap enough that I never have to worry about losing one of the pens. I can afford to buy 12-packs and have 3 pens scattered around the house + a stockpile of extras.