I'm not sure about dawnguard yet.
Is any of your characters a vampire or a werewolf? If yes, then the DLC is for you.
If like me you aren't interested in vampires in the slightest, you can still enjoy Dawnguard - the Forgotten Vale is a real treat, one of the most beautiful places in Skyrim. Add that one memorable sidequest, the old friend you can meet, very nice new armors and crossbows, a follower with fleshed out personality and backstory...
I keep a pretty healthy stock of saves so I'll be sure to be ready. How are the crossbows? Are they better than regular bows?
They're stronger, yes. Slower to reload, but it's not much of an issue, especially if you're sniping - my archer keeps using bows simply because I like them, but crossbows go extremely well with my warrior.
@MerchantMudCrab
You do realize he said "no spoilers", right?