assassin is a class which rely on surprise attack's and critical rate/evasion to survive and kill since is not naturally strong.

good stuff about being assassin -
easily kill any cloth/leather wearer's.
high evasion makes it easy to survive in pve and even be used as an off-tank when you have a good healer.
stealth makes it easy to survive in the abyss when you spot a group of enemies.
fast running speed makes it easy to escape mobs.
in group pvp with a healer assassin is very powerful due to short-recharging skills and stuns.

bad stuff about being an assassin-
at lower levels, both your crit rate and evasion is low so you will take long to kill and will take alot of damage.
if spotted first in pvp by a ranged mage/ranger you will most likely die so you have to be aware of your enviorment 24/7.
have no win to win 1v1 against a templar.
sometimes you wont be able to finish cleric with chain armor and shield in a combo,then he can just heal himself up with fast heals and turn around the tables of the fight.