We played with only 4 PCs, but with experienced players, and had no one go down at all (not even unconscious). We had the dwarf fighter, cleric, mage, and halfling thief. We did all 5 encounters (the table next to us skipped the undead room), taking 5 hours.
I played at the same table as Tony Vargas above, and while he thought it was fiasco due to the newbie DM, I thought it went fine, but agree that the DM's inexperience got in the way and it would have gone better with someone else. The DM plays in my group every week, so I knew what to expect, but Tony didn't, so I understand his disappointment. But every DM needs a first time, though in retrospect, I should have suggested that he do an Encounters or one of our weekly sessions first... (Although my first time wasn't too bad since I was DMing a group of newbies at just one encounter per week, so there weren't expectations.)
Overall, I thought that the cleric "war priest" was the most useful character, and the one that was most necessary to have in the party, for a variety of reasons.
I ended up with the wizard / mage, and thought he was kinda a lame character at first (and considered switching to the human fighter), but then he ended up being really useful. His Beguiling Strands blast 5 attack was quite useful for the slide 5 side-effect, the daily Flame Fountain had a perfect use in the adventure, and the magic missile was perfect for dealing with minions at range (as one would expect). When I play, I like to play melee characters (Warden is my favorite class), but on Game Days, I try to play something new, so the wizard was a good choice for me.
For me, there were two points where the wizard really shined.
The first was in the lower room with the orcs and the runes on the floor. I did the first attack with the blast 5 and then slid the ones affected to spaces with runes in them for additional damage. The next round someone said that since the runes triggered when one entered the squares, I could move the enemy onto multiple rune squares. (Although Tony had said that moving them back and forth onto the same square was too silly to be valid, so I resisted doing that.) I was even able to move one of them onto 4 rune squares with a 5 slide, for lots of damage, sadly ending in its death. (Boo hoo...)
The second was in the lower room with the undead. Right when we went in, the other PCs delayed and I immediately used my daily Flame Fountain to create flames blocking the doorway which last until the end of the encounter. So, the zombies and skeletons quickly entered the flames, taking 5 points. The fighter and cleric then hit those that they could. On my turn, I then used the blast 5 attack, hit nearly all of them, and then slid them all 5 away. On their turn, they move back into the flame for 5 more damage. Fighter and Cleric hit, I them move them away again, and they then move back into the flame. That continued until they were all dead, when they rise, immediately take 5 more fire damage, and then vaporize again. The party probably only took 1 hit in that entire encounter despite the 6 or so zombies and 6 or so skeletons.
The last room basically turned into the wizard hanging back and taking care of the minions while the other 3 handled the dragon and two toughies. We were thinking that it might end with a TPK after the first round or two, but it quickly turned when the dragon wasn't as tough as we thought it would be after its horrible breath attacks. I had the wizard use the slide on the dragon twice, where I discovered accidentally that sliding it into the brazier square gave it extra fire damage (and burned the cleric). So in a later turn, I did that again (but while intentionally missing the PCs).
In all, I think that the wizard made a huge difference in the adventure. Had I chosen the other fighter or other rogue, I could certainly have seen us as another TPK in the final battle due to damage taken earlier leading to at least one player death.
But most important, I had a good time with the introduction to the new game system, and that's the most important thing.