For me, the Vampires game pack was 100% worth the money, but I guess it depends on your tastes because I am a big fan of vampires (especially The Vampire Diaries) and of supernatural creatures. The pack really does it's jobs, vampires have awesome abilities, but playing with them too much will make it kinda boring, especially when some of the weaknesses make it a PAIN to drink plasma. Also, it's fun to have at least one vampire in your household and if you get bored of it, "store" it in a coffin with the "hibernate" option. That way, they'll only wake up when you tell them to and their needs won't decay at all.
If you don't want vampires to break in (or to at least NOT drink from your sim), place some garlic. It will give them a debuff that won't let them feed, and if your sim resists to a vampire, they'll get a 2 hours moodlet that won't let vampires drink from that sim.
I say it's worth it. The items are pretty great too.