I have always gotten liquid dishwasher detergent because it works so well. Then I heard a great tip that can save you a good bit of money in the long run while still getting your dishes clean.
Most powdered dish washing soaps are cheaper than their liquid cousins. The biggest problem with them is that they tend to leave soap scum and a white film on all your dishes. Well, there is a solution to this!
First, get out an old (but clean) ice cream bucket. I use these for everything by the way! Measure out 4 cups of the powdered dishwasher detergent and pour into the bucket. Next, measure out 4 cups of baking soda and pour into the bucket. Get out an old spoon (I use a metal one that has been bent a few too many times from dipping out hard ice cream). Stir the two powders together, not too quickly or you will send a fine dust into the air that will make you sneeze! :o) Put the lid on the bucket and store under your sink like normal. (Make sure you label the bucket!)
Next time you do a load of dishes, use the spoon to dip out what you need into the two soap cups in the door of the dishwasher like normal. Then, and this is the secret to not having the soap scum, pour about 1/2 cup of white vinegar into the bottom to the dishwasher. At this point, I just eye ball the vinegar.
The vinegar will react with the baking soda (just like in your 3 grade science fair volcano project) and will rinse your dishes clean.
The baking soda will make the washing detergent last twice as long, for half the cost, and still get your dishes clean!
You can buy large boxes of baking soda in the laundry detergent aisle. You can also buy 1 gallon jugs of vinegar in your favorite super store (you know which one I am talking about). It is in with the regular vinegar, just look down. :o) Oh and it's super cheap! Like less that $2.00!