Before my Pentium D (notorious for getting way too hot) with stock heatsink and fan was idling at 60, stress at 75, until it reaches 77 and turns off, and I used AS5. Obviously the heatsink and fans needed some cleaning, because they used to idle high 40's...

BUT!

I decided to clean it out and reseat my heatsink yesterday, and I picked up some Ceramique (for the price)... And my machine is idling in low 20's! I'm super excited.

I highly recommend this product :)

I'm wondering though, what exactly is the difference, and do other people get the same results with ceramique as opposed to as5, or what exactly are your preferences?

The difference may have been a seating problem before, or the old thermal paste dried out, or was too thick or too thin. Thick thermal paste can act as an insulator. It must be as thin as possible while still providing constant contact.
Or the computer have have moved something out of best position during a move or house cleaning.

wondering, what's the best place to store my unused compound?

just a cool, dry place with the cap on.

Check out this thread Zabean you might be surprised about Pentiums not always running hot.
Look here, www.techspot.com/vb/topic97942.html

I use high quality thermal grease not arctic silver and my temps with a oced Pentium D are 37C idle,50c load and I use a small case.

Glad your system is running well.