Dark Eldar are very fast. They hit hard, but tend to have problems with endurance. Their highest AV is 11 (except I am not sure about ForgeWorld). The majority of their units have 5+ saves and toughness 3. a lot of their units also have FNP.
The majority of their infantry weapons are poison so they always would on a 4+. You will probably see a lot of Venoms. They are 5 man transports that can put out 12 shots a turn just from the vehicle and has a 5+ invulnerable save. A venom with no one in it is only like 65 points. I would also watch out for Reaver jet bikes. They can be pretty deadly in assault and can even deal with some vehicles when they have the right upgrades. Most of the time you will be outnumbered with the amount of units they can put out due to the low point costs of most of their units.
Grotesques and Talos are probably their best assault units. Incubi are sometimes seen. They carry AP2 assault weapons, but are a little expensive for their survivability.
Some people also run what is called "Freakshow" lists. These are lists that make use of leadership deductions to cause damage or unit loss.
Dark Eldar are also frequently allied with Eldar, but with the new FAQ that might changes since the fast Dark Eldar transports were usually used to get Eldar Fire Dragons to the enemy quickly.