The Gauldur Blackbow is leveled, so it should be relatively on par with your current weaponry regarding damage (Before Fortify Smithing shenanigans). The enchantment also scales with your level, so that's nice. However, absorbing magicka is a niche weapon effect that's useless if not fighting mages.  I find it to work best when you have level 100 Enchanting and can put two enchantments onto weapons and armor. I like the absorb magicka when I run into mages and DOT damage like the shock effect that hurts everyone. Since you can't re-enchant any items, the Gauldur Blackbow is only mid-tier to me.