Most 40K players round here are Nid players, it's quite funny seeing about five different coloured armies with different units... in the same poses. Apart from the guy who has rippers bursting out of guard... but I digress.
A lot of DE players will make Dark Riders out of Wood ELf Glade Riders and spare warrior bodies.
Be careful about how many units of cold ones you intend to take - the more you have the mure DUR HUR moments you will get
Miscasts can be a masive pain, there is nothing like the elation of rolling two sixes then realising the other two dice are ones.:love-over:
Power of Darkness can be awesome, but dangerous if you miscast, if you have more than one Sorceress it's best to cast PoD with one, use her spells, then move on to the second Sorceress. If you get all your PoD out the way first you're leaving yourself with a sorceress surrounded by goblin boxers. We're squishy.
Also don't forget the other lores! We get PoD whatever lore we take (I believe, don't have book in front of me) and the other three lores in the main rulebook can be a blessing against some opponents.