This is true xD You were my first Sensai ever! And Katon is still my #1 specialty all this time later.
OK then, lets see what we got here...
Sloth slaps a blob of natural energy around someone for 2 turns cutting their speed in half as if they were running thru water, perfect to hit someone with who is quite speedy or just dropped their leg weights or used a speed enhancing jutsu. Also I'd use it on someone who was attempting to fight close quarters giving me an edge in speed/reaction.
Greed, while a basic technique, is quite fabulous in my book. Using natural energy to increase muscle mass adding +20 to Tai, increased strength and 1 1/2 times speed increase, and not affected by turn restrictions. Whats not to love! The possibilties for use are quite large, too large to go into right here as I could type for a looong time xD.
Envy is like being able to look at the enemies jutsu and be like "Talk to the hand", ignoring it and sending it back to it's origin. Of course it doesn't work on genjutsu (would be cool if it did) or on omni-directional attacks, and you have to be able to detect it coming, but still kinda cool to just send someone's own stuff right back in their face.
Lust is just Geets being Geets, his handsomely manly form being enhanced by senjutsu chakra making women and men alike go ga ga over his powers of handsome manliness. Dive into the ocean blues, but I warn you..the water is deep 0_0. -10 to their techniques, slowed down reaction speed by 1/4 all for 4 turns, and the only way out? You gotta hurt yourself in a manner that would break you out of an S ranked genjutsu. Pretty cool ability.
And last, but certainly not least, we have Gluttony. Using massive amounts of natural energy one can add a whopping +40 damage to any technique it's sent into afterwards. This one is a game changer and can really turn the tide in a battle. One must remember though that it only works on elemental attacks, that's the only downside to it, though there is no real downsides to any of the Sins.