Are coffee grounds good for plants?

Yes they are excellent choices as organic fertilizers.

If you use them, use less or no synthetic (commercial) fertilizers. Since coffee grounds lack phosphorus (the key ingredient in fertilizers for flowering plants) it is better on non-flowering shrubs,
foliage plants and grass than flowers. However, if used in combination with phosphorus (e.g. super phosphate or rock phosphate) it is excellent -- and cheap. You can buy super phosphate and rock phosphate at garden centers and home improvement centers.

Used coffee grounds contain about two percent nitrogen, about a third of a percent of phosphoric acid, and one percent potash; DRY WEIGHT. If you could buy coffee grounds in bags at garden centers the 3 numbers on the bag would be 2-0.33-1

It is not particularly "slow release" as typical of most organic fertilizers. Most of the nitrogen will be available pretty fast. This means you have to watch out not applying too high quantities at one time, and that more then one application during the growing season may be needed but in smaller volumes at a time.

Analysis of coffee grounds also shows that they contain many minerals, including trace minerals and carbohydrates. This makes it a good fertilizer.

Since coffee grounds are acidic they are particularly useful on those plants for which you would purchase and apply an “acid food,” such as

blueberries, evergreens, azaleas, roses, camellias, hibiscus, hydrangeas, gardenias, etc.
Most edible garden crops prefer slightly acidic soil as well.

They would not be ideal around plants like lupines which require a sweet soil. If your soil is naturally alkaline (e.g. pH higher than 7.0) using coffee grounds is extra beneficial.

Since coffee grounds tend to get moldy, they should be incorporated into the soil, or dried and used as mulch. If you have problems with too high pH, the use of coffee grounds are a good solution.

You can also dilute the grounds with water, and water the plants with the "tea" created. This is a good way to get plants growing immediately. Just avoid using around very small seedlings as it can burn the young plants. for established plants it is great.

Dose:
use about 6 table spoons per plant dug in near the roots, or in pots. You can also mix in a 1/4 - 1/2" layer and mix it in thoroughly in the top of the soil. The coffee grounds will help aerate the soil and provide nitrogen and potash to the plants. The phosphorus, should be added separately, however, such as rock phosphate (0-4-0) or super phosphate (0-20-0) available at garden centers.

Here is another thought: Ants do not like coffee grounds, and it is said coffee grounds
repel snails and slugs.  

"The best fertilizer is the gardener's shadow"