Pros: powerful ultimate, mist form gives templars an escape ability which they otherwise lack (except it's currently kind of broken)
Cons: Fire vulnerability, health regen, and Fighters Guild abilities destroy you in PVP

My main is a magicka templar and I cured my vampirism after trying to make it work for so long. There are just better abilities to use for now, but I think with the DB update coming and the changes to vampire it might be a good idea to reconsider playing as a vamp again.

Speaking as a mostly-pve (for now) stage four vampire dragonknight healer, I personally like the pros more than the cons of being a vampire, and believe it will only get better with the Dark Brotherhood update.

The lower health regeneration takes a bit of a playstyle change to get used to, but isn't a big deal at all. As a healer, I am used to watching everyone's health bars anyway, so I just have to be more productive about my own, too.
The fire weakness takes more getting used to, but again, not a big deal. I love dungeons, and the bosses love fire attacks. I have always been very mobile with my healing, so I just had to make a minor change to my playstyle to keep a bit more distance from the bosses with heavy fire attacks, and play smarter with my shields (group and personal). I also have a spare necklace with fire resistance instead of the spell power enchant on my main necklace... But to be honest, I have not used it yet? I have some CP in elemental defender, but not a lot. Maybe 10-15? I prefer to throw my points into Bastion. In PVP, fire-based siege weapons will mess me up, but I survive them more often than I don't.

For advantages, the regen boost to magicka and stamina is really nice, and after Dark Brotherhood it looks like I won't even need a skill on my bar to get it (which is good, because I frequently run without one, although I do love the bat swarm ultimate).
I am also a giant fan of the undeath passive, as it gives me extra time to keep myself alive and heal our whole group when we all take a big hit from a boss.
And the sneaking passive! I fell in love with sneaking when I turned vampire.
I may be in the minority, but I also love the elusive mist skill. While I don't usually have it on my bar, I have used it to great effect in dungeon PUGs where the "tank" is actually a dps who wanted a shorter queue time. I use it to "tank/kite" melee bosses that would one-shot my light armored self if I end up playing healtank.

I still have a lot to learn about PVP, so I can't speak much about that side, except to say that so far the fighters guild abilities have never made my death recap list.
The sneaking passives are also really nice, in my opinion. I didn't sneak much before I went vampire, but afterwards I am loving it, and putting all the points I can into sneak related passives.