The good & The bad: Express your joy, your disappointment

The good

Una gozada [Familiar] This noun applies to anything that's a very pleasant experience.
Example: Es una gozada verle jugar [Familiar] It's a pleasure to see him play.

Guay [Familiar] Cool. This is a catch-all adjective to describe anything that's cool.
Example: ¡Mira qué zapatos más guays! [Familiar] Look at those cool shoes!

Guay del Paraguay [Familiar] Lit. Cool from Paraguay. As cool as it gets - a silly rhyme that reinforces the idea of cool.
Example: El móvil que se compró está guay del Paraguay [Familiar] The mobile phone he bought is really, really cool.

Alucinar [Familiar] Lit. To hallucinate / to trip. It assumes the object you're praising has had such an impact it is comparable to a hallucinogenic drug.
Example: Se compró un coche de alucine [Familiar] Lit. He bought a car that makes you trip. He bought an incredible car.

The Bad

¡Mierda! [Very Familiar] Shit!
As in so many other languages, when something goes wrong there's an immediate cry announcing excrement.
Example: ¡Mierda, he vuelto a perder las llaves! [Familiar] Shit, I've lost my keys again. mierda [Very Familiar] Lit. ...of shit.
From that follows that whatever's gone wrong is made of excrement.
Example: ¿Dónde están las llaves de mierda? [Familiar] Lit. Where are the keys of shit? Where are the damn keys?

Una mierda [Very Familiar] Lit. A piece of shit. In Spanish you actually quantify it.
Example: No me acuerdo porque tengo una memoria de mierda [Familiar] I can't remember because I have a shitty memory.

Chungo/a [Familiar] Dodgy.
Example: Es una tía un poco chunga [Familiar] She's a bit of a dodgy girl.

Hortero/a [Familiar] Tacky or gaudy. You may also hear hortera, ending in -a, even when talking in the masculine.
Example: No te pongas esa chaqueta tan hortera [Familiar] Don't wear that corny jacket.