So im doing the a quest where i have to kill this vampire Hern. I got infected but cured it with my potion. Im wondering though is it cool to be a vampire? Are there any big downsides?
the downsides, as well as the ups I suppose, really depend on the stage of vampirism you are at.
For all stages the up sides are
Resistance to diseases, resistance to poison, and a 25% boost to sneak and illusion skills.
On the down side you will have a weakness to sunlight, this means your stamina, magika, and health will not regenerate while the sun is out.
Stage One Vampirism gives you a weakness to fire, and a resist frost. Along with a reanimate dead, health drain, a detect life powers.
Stage Two gives an increase of 25% to all stage one effects in addition to vampiric charm power.
Stage Three again sees a 25% increase to the effects of stage 2, now landing all affects at 75% with the exception of the 100% resist poison/disease. Stage Three adds no new powers but instead beefs up the life drain and reanimate dead powers.
Stage Four all effects are at 100%. The spells are again beefed up and you are given the Embrace of Shadows power, allowing you to see in he night and turn invisible for a time.
Whether or not this is worth it is all up to you, imo its worth trying at least once though reversing it is sure to be a pain in the @@@ as it has been in all other elder scrolls games though I have not done it in Skyrim myself. The weakness to sunlight thing gets annoying if you find yourself without pots to make up for the lack of regen but other than that the only downfall to being a vamp is that townspeople will recognize you for what you are an essentially attack you on sight. This becomes a problem for bartering and really restricts you as a character to the underbelly of Skyrim, though for me this is the prefered choice anyhow so not a huge loss.