I've been playing it off an on since release. Its the only game I keep comming back to.
Its excellent! It is for the *hardcore* MMOG players amongst us, due to its complexity.
1) Class structure
2) Tons of Skills to understand
3) The myriad of Implants and clusters
4) A crap load of damage types and weapons, and armors that take advantage of all of them.
5) Spell system (nanos) is insanely difficult to grasp due to the naming of them.
6) Tradeskill system is difficult to master since you actually have to learn practical scientific terminology.
In fact, the names of everything (from weapons, armor, nano's (spells), implants, clusters) are based on real scientic theory... not just 'Healing Spell 1' 'Damage Spell 1'. heheh
I mean, the list goes on and on. You could easily sit for 3 hours and do nothing but tweak your character out in armor, implants/clusters, skills, nano's (spells), and weapons.
Not to mention Gridstream! Listen to great music from 'inside' the game (ShoutCast), and give requests to the live DJ via in-game tells.
The game is clever, smart, and well worth the time to check out.
Again, this game is for the hardcore RPGer amongst us.