Seventh chords are useful for many different styles from rock to jazz, country, folk, and even metal. I love rock tunes that incorporate some funky/jazzy/mellow/emo seventh chords rather than all powerchords all the time. No matter what style you like best, it's good to know at least three different versions of the main types.
My chart of seventh chords is a good way to learn them. Each of them have the root note (the same as the name of the chord) on the bottom so you can find them easily. There are three different ways to play each type of chord, one for the E, A, and D strings.
Finally, remember that all these chords are MOVABLE, which means you can play them anywhere on the neck. There should be no open strings sounding, so block them out.
It's a lot of memorization, but if you can remember this chart you will be set on progressions with seventh chords. In the theory section I'll cover where these chords come from and what they sound like.