I was planning to go with the Dawnguard, but for whatever reason I was very interested in learning more about Serana, and I figured that I'd have to join the vampires to do that. No regrets. Vampire Lord is amazing, and I've got a character that's a perfect fit for the Dawnguard to play later.
Also, it bears noting that at one point, as a vampire, I returned to Fort Dawnguard to see if anyone sold Crossbow bolts. No one did. So I talked with Isran a bit, and he failed to notice my glowing vampire eyes and fangs. As he lectured me on the importance of keeping your eyes sharp and on the lookout. Then he told me that sleep is for the weak, as he climbed into bed and shut his eyes. I didn't really want a drink, but when someone is asking for it THAT bad, how can I say no? Most incompetent vampire hunters I've ever seen.