After watching quite a few on-line reviews picked RA as the deckbuilding game of choice for my play group. This is what we, as a group, have discovered so far:
Humans are, hands down, the best race in the original AND the expansion. Because of a little something called "card advantage". FOUR out of six cards in their barracks draw cards in one way or another, they have the second hardest hitting 6 cost unit in the whole game PLUS archers, that are incredibly, amazingly, stupidly powerful in a siege. You just keep on playing and drawing footmen until your opponent has played all his cards, then BAM - you kill his most scary dude. He has now lost a whole turn's worth of his game and some life/a neutral city to boot. Once you get the hang of them, humans can start hitting for 9-12 on TURN 3-4! How nasty is that?
Undead are a close second - they can, and on a regular basis do, take down 4 or 5 cost cities on TURN ONE. Balancing the number of Animates you wnat in your deck can be problematic and one poor draw at the wrong time can screw with a perfectly good game plan, but the race's still awesome. Especially with vampires. Vamps rock.
He Uthuk are also capable of crazy combos - wound an opponent's fatty then devour for my obscene? You're down 3-4 might, and I'm up by 4? Welcome to the jungle. Problem is, that's the only combo that really works with these guys. And it's incredibly difficult to set up. Sure, you can get rid of any units you don't find useful any more by wounding them, thus drastically increasing your chance of getting the three cards you need in your hand more often. However, by the time you have all the pieces in your hand, an undead or human player would have eaten your guts. Or overrun you with footmen and knights, respectively.
Dwarfs are... Awkward. They depend on having gold in their deck, but drawing it just on time. If you draw too few or twoo many coins at the wrong time you're in for a rough time. The warmachine looks cool and useful, but in practice all it does consistently is destroy coins - usually there aren't really all that many units it can give bonuses to. I've also found that the much admired demmolitionist is extremely counterproductive - it thins out your opponent's deck and can't even destroy his more valuable resource - the 3 GP coins! Damage potential, I've found, is extremely limited with these guys.
The Orcs are even weirder. Most of their units appear cool on paper, but don't really work! Their first two units seem OK, but the only unit that gives them ANY card advantage has an incredibly crippling precondition. Lizard rider is just plain dumb. The troll is menacing, but ultimately doesn't have as much of an impact as could be expected. That "purchasing in combat" trick is really cool, but ultimately - difficult to set up. And if attacked while they don't have it, orcs'd probably take a serious beating. Their main advantage, I'd wager, is in their ability to attack 6, 7 and 8 cost cities on turn 2-3 and then use the influence to buy expensive units off the open market. However I don't think that's ever worked out for anyone I know.
The Elves look really cool. But turn out to be so incredibly, agonizingly SLOW. They have the best creature destroy in the game, and the pegasus can dredge up their deck for the creature you need to win any engagement, the forest guardian is a killer in and of itself - in short, they have a killer late game... But they never get there. The humans start hitting for 12 at turn 4, remember? The elves aren't aggressive enough to get major cities and use their archers to full advantage mid-game, and the game doesn't usually last long enough for them to really shine. Which is sad.
All in all? Great game, fun all around, now if only humans didn't bash our heads in every time we played competitive...
As a further note, I tried fixing the obvious imbalance by adding a 2 gp coin in place of the 5th 1 gp in the starter decks of of the elves, orcs, uthuk and dwarfs. I saw it somewhere on this forum and it seemed like a reasonable enough idea. This sped the "lesser" races' game considerably, but it quickly became obvious that it did not make them broken. The house rule managed to even their odds against the Undead, but not against the humans. Humans are still overpowered, and there doesn't seem to be a way around that.