I tend to always go for the original presses due to reasons already mentioned in this thread. One thing that really pisses me off about some reissues is the unnecessary inclusion of entire albums on the same disc, with the front cover depicting smaller versions of the original artwork. I'm glad to say I own none of these and I refuse to ever buy any.
Particularly with Old School Death Metal, I like to have the originals not only for the music and original artwork, but as a piece of Death Metal history. These were all worth shelling out extra cash for (or simply persevering and hunting down second-hand copies while reissues were available):
Demigod - Slumber of Sullen Eyes
Miasma - Changes
Gorement - The Ending Quest
Beherit - The Oath of Black Blood
Sacramentum - Far Away from the Sun
Varathron - His Majesty at the Swamp
Therion - Beyond Sanctorum
Sinister - Cross the Styx / Diabolical Summoning
Adramelech - Psychostasia
Unleashed - Where No Life Dwells / Shadows in the Deep
Darkthrone - Transilvanian Hunger / Soulside Journey / Under a Funeral Moon/ A Blaze in the Northern Sky
I desperately want the original of Carbonized - For the Security. The reissue is among the worst I've ever seen.
Edit: And the fucking original of Merciless - The Awakening. On the reissue the spine reads: MERCILENSS - THE AWAKENING.