I have used Manic Panic many times on my (dark brown) hair, and have always been very pleased with the results. It is true that you should bleach your hair to within an inch of its life if you want the color to show up, but personally I didn't notice too much damage as a result -- in fact, I have read that the cream dyes have conditioning agents in them to counter the effects of the bleach. The cream dyes can be somewhat time-consuming and messy to use, because you have to rub them vigorously into your hair until the dye foams, but the colors can be very beautiful. I have also noticed that blue-toned colors are much more difficult to pull off, mostly because unless the bleached hair is practically white the end result will be a sort of turquoise-y green (not unattractive, though). And, as the dye starts to fade, sometimes the color will look a little dingy and brassy. Thus, pink, red, or purple is usually easier to pull off. Oh, and Vampire Red seems to get a bad rap, so if you are looking for a red dye I would suggest Pillarbox Red, which was the very first Manic Panic dye I ever used (aww) and produced a nice, deep red.
Here is a complete list of all Manic Panic cream dye:
After Midnight Blue
Atomic Turquoise
Bad Boy Blue
Black and Blue
Cotton Candy Pink
Deadly Nightshade
Divine Wine
Electric Banana
Electric Lava
Electric Lizard
Electric Sunshine
Enchanted Forest
Flaming
Fuschia Shock
Green Envy
Hot Hot Pink
Infra Red
Lie Locks
Mystic Heather
New Rose
Pillarbox Red
Plum Passion
Pretty Flamingo
Purple Haze
Raven
Red Passion
Shocking Blue
Tiger Lily
Ultra Violet
Vampire Red
Virgin Snow
Wildfire