My gaming group and I played the first edition of Descent a lot, including the Road to Legend campaign all the way through a couple of times. When this second edition came out, the logical thing to do was to buy it. I'm pleased to say that it not only met, but exceeded out expectations when it comes to what a good dungeon crawl should be. It's better than the first edition for a variety of reasons.
First, it's much more accessible. FFG did a good job of reducing the amount of overall rules and significantly shortened the play time. Going through a quest now only takes a couple of hours, whereas with the original Descent it could easily be double that, depending on how the Overlord played. I can see the second edition being much easier to pick up and play, even if you never played the first edition. Revising the line of sight/monster spawning rules was a very good decision on the part of FFG too, and is one of the big reasons the game doesn't take nearly as long to play.
Second, it's a lot more story-driven. In first edition, the victory conditions were usually tied in with destroying some big, bad monster. In second edition, different quests have a large variety of different victory conditions, and it really makes it a lot more fun to play overall. Heroes actually have to think about how to win rather than just mindlessly hacking at monsters, which is nice. The new hero attributes system makes the game much more role-playing-ish, which is cool too. I very much enjoy the new terrain tiles, as well.
Third, having the heroes divided into classes is a nice touch. I'm happy that they actually included a healer class in this edition, something that I always felt the first edition was lacking. The only class changes I'm not thrilled with are those of the tank class; for one-shot encounters the tank is almost as easy to kill as the mage. If you play the extended/epic variant with more gear, that problem does alleviate itself, but I still feel like FFG could have done a better job with tank class design. Allowing any hero to equip any gear is kind of lame, but the amount of gear that comes up in a campaign is not huge, so I don't see it as game-breaking by any means. Overall, the classes work well and are tuned well, and the Heroic Feats are a cool mechanic (though some are noticeably stronger than others).
Finally (and most importantly), it's a lot more balanced. First edition was -heavily- weighted towards the Overlord if the Overlord knew what he was doing. My group and I had to institute a huge number of house rules in order to balance the game out. The second edition is much more even overall, and we have not had to institute any house rules for anything other than stylistic reasons to this point.
Overall, we are very big fans of second edition, and it's endlessly customizable, so when we finish the campaign that came with the game I can easily design more. It's certainly more of a tactical combat/dungeon crawl than a RPG, however, the RPG elements it does have really help the game come together nicely. Being able to buy a conversion kit and use all the monsters/heroes from first edition is a helpful, too, though I can see people enjoying the game even if they never played the first edition at all, using just the included monsters/heroes. I honestly can't really see us ever going back to playing straight-up first edition, although I may use some of the stuff from it for future campaigns I design. Kudos to FFG for designing such a great game!