I would pronounce it JAH-ciel (ciel like the word 'sky' in French). I would not be surprised to find it's pronounced YAH-ciel, as I would assume the little Jasiel was hispanic. I really like it, but then again I like most of the -el names, especially the ones that sound a little more made up. I like combos like...
Jasiel
Anthony Jasiel
Romeo (prn ro-MEH-o)
Jasiel Natán
Jasiel
FranciscoJasiel
Enrique