I'm currently a level 18 Des/Con/Alt Mage and was thinking about becoming a Vampire but not sure if it's worth it. From my understanding you lose regeneration for both magic and health but items that give regen still work? I have Dawnguard but plan to stay away from it until I'm done with Skyrim's quest. Any added insight or tips would be really helpfull.

As with everything it will depend on your playstyle as to whether or not it's worth it.

I play a Vampire and find it very powerful. I assessed my weaknesses and used my strengths against them. ie: I made gear using the Necromage perk so I had powerful enchantments of Resist Fire and Health/Magicka/Stamina Regeneration. I like to stay at level 1, so the need to regularly feed keeps me aware of the in game time and whether it's day or night. For another player that limitation might be annoying.

Ultimately, being a Vampire or not should probably come down to how it suits your RP.

I did it on my evil character for RPing purposes only. It's cool, but definitely not for everyone. For mages, though, I'd try it.

You lose 15-60 points Magicka, Stamina, and Health (Depending on the Stage You're In), and they don't regenerate. (NOTE: This is only when outdoors during the daytime. To counter-act Lost Magicka Regen, you could play as an Altmer and use Highborn)

You gain increased resistance to frost, and increased weakness to fire. This is in effect no matter where you are. To counter-act this, you could play as a Breton due to their natural fire resistance (IDK what race you are)

Items that regenerate Health/Magicka/Stamina faster still work. For example, if out in the sunlight with the Ring of Recuperation (Stamina Regenerates 20% Faster), Stamina regenerates at 20% of the normal speed.

You can always try it, and if you dislike it, complete the Rising at Dawn quest to cure it. If you have any other questions, let me know.