Grinders:

The most important piece of gear a coffee lover can own, and should not skimp on, is the grinder. The quality of the grinder is almost more important than the quality of the brewer. A poor grind is inconsistent. The particles of ground coffee are different sized; some particles will over-extract when brewed, some will under extract. Neither is good, and you will get sour, bitter, nasty coffee. (Just because your coffee isn't outright nasty doesn't mean you shouldn't buy a good grinder; like I said, your coffee can ALWAYS be better). Whirly-blade, or nut grinders, are the worst in this area. They pound the beans into bits, cave-man style. If that is all you can afford, no problem, but if you can pony up at least $40, you should.

Burr grinders are best. Cheap burr grinders grind into consistent sized particles; expensive ones actually shave slices off of the beans. Once you dial in the right grind for your machine, all the precisely ground particles will extract evenly, and you'll have better tasting coffee.

Back to the whirly blade: any grinder is better than no grinder. Buying pre-ground beans is the easiest way to serve bad coffee. So if the whirly-blade is all you can swing, do it!

Entry level: Capresso #555

Mid-level: Baratza Virtuoso

Top end: Rancilio Rocky (for all grinds, including
espresso)

Espresso only: Mazzer Mini