Gareji is not really a name you want use for shako unless you want to sound like a middle-aged, tired oyaji. :D
Shako is a homonym for a car garage, so oyajis think it's really funny to call shako "Garage/gare-ji"...
I've always had it boiled - I think it tastes like an overcooked shrimp...
I can't remember if I've had shako at Ino Sushi in the past 6 years, but definitely have had it at Sushi Tomi in Mountain View and Sushi Sam's in San Mateo (which seems to have it every year).
Both places where I've had shako served it as already cooked, served a little bit colder than room temperature, and brushed with nitsume or is the right word tare. Very chewy, muscular, or shall I say "gamey seafood". In some cases the chewiness was like an overcooked old lobster. Not quite worth the $6 to $8 for a nigiri pair :-/ which would be better spent on other sushi neta. I'm sure it tastes way better in Japan, just like kurama ebi.
Umetaro is right, in Hong Kong these are called "lai liu ha" or pissin' shrimp (with peeing referring to wetting onself uncontrollably). My uncle used to drive an hour+ to his favorite lethal street food joint in the 80s, and ask for a plate of them, stir fried sale + pepper (jiu yeem), no doubt with the "1000 year old re-used oil". I'm sure that contributed to his high cholestrol levels, but he loved it. Apparently the Hong Kong shako's have less meat in them, unlike their Japanese counterparts.
re: K K
The muscular taste doesn't surprise me: while I have never eaten a mantis shrimp, they are very, very strong, and indeed, are probably one of the strongest animals for their size in the animal kingdom...the force of one striking is equivalent to a 22 caliber bullet, and they can easily knock off a finger or two off of a careless diver with their 'kick'...
here is more than perhaps you wanted to know about these fascinating creatures: