Does anyone actually use the buff moves like Growl, Leer, Agility etc? 

Every Pokemon game that I've played I have never ever ever ever used the buff moves and managed to complete them with no trouble at all. Ryan said during the Pokemon Black quicklook that he was the same and always opted for moves that dealt out damage for the sake of not wasting a turn. 

Are the buffs aimed towards playing against human opponents? I have no experience playing online so I don't know how the tactics work on a competitive level.

Growl and Leer? No. But yeah, I use buffs. The good ones. Sometimes it is better to buff for a turn and then deal more damage the second. Stuff like Water SPort? Never.

I've never used them either, so I have no idea.

I'm sure they are used in the metagaming/competitive scene.

The good ones, yes. They're important to set up sweeps and other tactics. I have a Zoroark sweep that relies on getting off a Nasty Plot while disguised. Another instance is my friend who has a Cloyster with Skill Link and Shell Break, which can sweep most teams easily. 
 
A good tactic for Shauntal (Elite Four) is to send out a Zoroark disguised as a fighting pokemon- she'll try to use Psychic to no avail, and you can pull off three Nasty Plots/Swords Dances and just blow through her entire team.

no i usually rather attack then was a turn lowing  defense or something

I'm having endurance run flashbacks.

" The good ones, yes. They're important to set up sweeps and other tactics. I have a Zoroark sweep that relies on getting off a Nasty Plot while disguised. Another instance is my friend who has a Cloyster with Skill Link and Shell Break, which can sweep most teams easily. "

I never use buff moves and that is why I suck. I just like to deal damage as fast as I can. I don't like using turns not dealing damage.

that dragon dance is a beast also, swords dance, calm mind, curse, ya know the ones that mean something more than lowering defense and stuff, I might use screech here or there for the luls though lol

Dragon Dance is a good buff, I use it with my Charizard. 

" @isnipeyoudie said:

" The good ones, yes. They're important to set up sweeps and other tactics. I have a Zoroark sweep that relies on getting off a Nasty Plot while disguised. Another instance is my friend who has a Cloyster with Skill Link and Shell Break, which can sweep most teams easily. "

All that PokePro jargon is wasted on me. :( "

There's versions of both Growl and Leer that do a super-drop of Atk/Def, however I usually only use buffs to my pokemon, rather then nerfing the opponent (only useful for his current Poke)
 
1. If I'm loseing against a E4 guy I don't have a counter againist, the  I'll try to max my buffs to sweep em out.
2. a Legendary I'm trying to capture is smashing my team with an attack. Usually then I'd try to lower his attack stat so my Pokes stop dying >_>

The only real buff/debuff moves I use on a regular basis are any Accuracy-Lowering attacks and anything that "Sharply" raises my stats. But for the most part, I find myself very attracted to attacks that both do damage and raise my stats/lower the enemy's stats, that's the way to do it!

Yeah I use Swords Dance and and Dragon Dance.  They are a huge help.

Swords dance, calm mind, agility, and bulk up, and the double screens are all commonly used for sweeping and defense. Debuffs aren't so useful though. Except for like, haze and things that reduce stat changes completely, or switch stats and the like. 
You have to remember, one swords dance effectively doubles your attack stat, and if you're using something like Rampardos and you get a rock polish on it (+2 speed) literally nothing is going to stop the thing.

No.  
 
I would use them if Pokemon had more than 4 moves at once.

I rarely use buffs and debuffs and also lean towards the "I'd rather deal damage" mindset.  The only ones I really use are ones that sharply raise or lower stats, or damage dealing attacks with them mixed in.  For example; Flame Charge, Iron Defense, or Charge (followed by an electric attack).

I never use de-buffs against my opponent, it's always better to just buff up your own pokemon to either allow them to take multiple hits or to increase their stats to deal major damage.

Yeah I never use De-buffs against my opponent either, not really worth it. If you're going to use up a move slot use it on a buff for your own pokemon. 

Playing Heart Gold I used a Staryu with whatever that move is that increases avoidance (edit: Brightpowder) + an item that reduces enemy accuracy. For some battles I would max out avoidance while healing with recover and then proceed to wipe the floor with the trainers pokemon without getting hit :D 
 
Probably not a viable tactic against a human opponent, takes too long. 
 
Sometimes I've used screech or Sword Dance on rare occations but as far as non-attacks my favorites are Sleep powder (or Spore, though it's a rarer move) and Confusion Ray.

Didn't use buffs or de-buffs at all. My tactic was to grind levels for each of my Pokémon