There's no denying the convenience of an already-ground spice, but I'm here to make a case for why you should consider adding a few whole spices to your seasoning line-up. Four words: better flavor, fresher longer. Convinced?!
One of the things that makes spices like cardamom, cumin, and nutmeg so very delicious is the oil they contain. These oils, both flavorful and aromatic, are extremely volatile when exposed to air, but stay pretty well protected as long as they remain inside the seed or spice. As soon as you grind them up, the oils are exposed and spices start to lose their awesomeness.
If you do a side-by-side comparison of, say, ground cumin from a jar and whole cumin seeds that you've just ground yourself, the difference is astounding. Pre-ground cumin will smell and taste...ok. Fine, even. But the freshly-ground cumin will have an eye-popping aroma and a deep, complex flavor. If you toast the spices before grinding them, then you'll get even more of an aroma and flavor knock-out.
You can grind whole spices in a spice or coffee grinder (get a second one so your spices don't pick up coffee flavors!), or you can use a simple mortar and pestle. This is actually my favorite way to grind spices, mostly because I find it easier to wipe a mortar clean than to clean a grinder.
If you don't feel quite ready to give up the convenience of already-ground spices for your weeknight meals, I'm totally on board with that. I do buy a few of the spices I use most frequently pre-ground, saving their whole-spice counterparts for weekend cooking and special meals. If I'm feeling extra-industrious, I grind a small batch of the whole spice every few weeks to have on hand.
One last note: although whole spices keep their freshness longer than ground spices, they won't last forever. They start heading downhill after about a year, but are generally ok for up to two years.
Do you cook with whole spices? Do you think it makes a difference in your cooking?