I ran with a group of 3 friends on release night. We defeated the login boss in under 30 minutes and rushed up with our group (1 monk, 1 barbarian, 1 witch doctor, 1 demon hunter = 2 tank, 2 dps)
We hit level 47 and cleared nightmare in 24 hours (including one 2 hour break for a short nap / server down). Then the group broke up (we had to go back to work, sadly). I should also note that I found an amazing shield around level 40 and have been using it since (huge +Dex & +Vita, on a rare).
The group strategy was me using cyclone strike to stack mobs, then AoE CC with blind. The barb followed up with an AoE stun of his own, by which time we could AoE down just about any group of non-blue/non-yellow monsters.
This was the build that I was running there:
I played for a few hours last night in Hell, solo (because they have cleared hell already). Now that I am solo'ing, I am still using a very defensive style. Here is my current build:
At the moment I am level 50 (almost 51). I am running all mixed Dex/Vita gear (mostly rares, some hand-me-downs from my Demon Hunter friend). While solo'ing I have used the Scoundrel follower for additional dps (Don't need the Templar for Beef, and did not try out the enchantress).