I would just advise people to stay away from the dueling modules... I sat around and built characters up to level 20 for dueling for about 6 months after I got the game, and believe me, reducing the game to a war of statistics is no fun. It's hard to find the same appreciation for the game after you've run the numbers and found all the most powerful combonations. Playing online you can only really expect to find players who power game, even if they don't mean to (i.e.: maximum hit points from reroll after reroll), so it isn't a very .... "relaxed" setting to explore. I'd suggest you tow yourself through the single player campaign first to get a feel for it, and move online if you're comfortable with it. All in all I'd say it's worth getting if you haven't tried 3rd edition yet.
Cheese, you're right, it isn't the same game, but it's pretty close. Don't expect the feats to match up or the spells to be bugless (or all present), but knowing how things are supposed to work helps to prepare you for the skills that actually follow the rules, and give you a better understanding of how the game mechanics are supposed to work together. Again, a lot of the discrepencies are only uncovered through powergaming, so if you're looking for a more relaxed challenge and you aren't abusing the system you shouldn't notice too many issues.