Addictive for those comic fans, but in the end repetitive.
Missions are hard enough and push people to gather in teams. Mission have a certain way to scale according to the people that does them (number of enemies changes according to level and number of people entering the mission), so it's absolutely awesome when you do a mission with many people (especially higher level than you).
Gameplay is varied (archetypes and power sets give out a good deal of customization and variation) and costume/avatar editing is simply beautiful.
Apart from this, I suggest highly having at least 1GB ram. COH is quite memory-hungry (but not CPU and GPU) and 512MB will slow down loading sequences and sometime playing also. GPU requirements are not excessive (compared to today's latest beasts such as Oblivion or Doom4) so you can play in good resolution (recommended 1024x768 or higher or else interface will be cluttered and chat screen won't have enough space).
Overcrowded areas tend to slow down if internet connection has high latency time, though bandwidth requirement is quite low.
Overall score says that you can have some fun, that if you like you can entertain yourself for a while, but the monthly fee may discourage some people after a month of two after seeing that repetition is around the corner.