Bad day or not, if the place isn't busy and the prices aren't diner-low, then the least he/she can do is keep my water/beer/iced-tea glass filled. A delay is fine if it's busier, but to finish a drink at the beginning of the meal and then not see your waiter or waitress until you've finished is unacceptable.
I've seen good servers handle many tables and I've experienced the disappearing act on relatively slow days. Each server is tipped appropriately. For great service, it depends on the bill and how much the server worked. If I go somewhere fancy and the bill is over $100, I won't tip more than 20% for even great service. If it's expensive and the server didn't have to do a whole lot of work, I might still go as high as 20%, but will usually follow JHZR2's method of rounding down to something closer to 18%.
If I'm at a diner and I see every one busting their hump and taking care of every one, I usually leave more than 20%. When the meal is only $20 for the two of us, what's another $5? I'm usually fine having my usual eggs and toast at home, so going out and having a substantial breakfast is more of an occasional treat, anyway.
For so-so service, I'm with JHZR2's, as well, at 15%. From there it goes down, but it's really tough for me to give less than 15%, except if the server was out-and-out rude. I'm all about yes ma'aming and siring every one I come across, and even if I'm in a bad mood I smile when greeted, so rudeness is unacceptable and leaving 10% or lower gives me no pause. More often than not, I've had good experiences, the bad ones being few and far between, albeit a lot more memorable.
The example that sticks out in my mind occured when I was maybe 15, down on Kent Island, MD, at a Golden Corral. The crabs weren't running, so my dad and I decided to let the traps sit for a couple hours while we went for breakfast. It was still early, so there were only two other old men there. The waitress brought us over cups and bundles of silverware and said she'd be back with the coffee. She served both men, one fifteen feet to our left and the other fifteen feet in front, toward the buffet. She never even came over to us. We grabbed our own drinks and the food was self-serve, so that was that. My dad held up his cup when she went around again and she simply ignore him. My dad left her a quarter and I a stick of gum. I never got my OJ that morning, but the apple fritters were delicious.