It has exceptional stats, you can parry, I switch it out for blue and red dragon mini bosses. But I love this shield +5.

It literally only has downsides if your a pure caster.

I'm a caster :(

It needs to be actively in your hands or on your back for the magic reduction. If you have a talisman or catalyst in your 2nd left had slot and switch to it, the effects of the shield go away.

If by "decrease magic power" you mean your magic damage that you deal with spells/weapons is lowered than no. The shield simply buffs your magic defense while it is visible on your body (in hand or on your back). It doesnt make you deal less damage in any way

It decreases your magic damage output, so no. You end up worse off.

It only decreases your magic damage output while in your hand, if switched out you still get the maximum benefit of the sharpness ring (you should be casting with kris in hand anyway)

They mean that it decreases incoming magic damage. In other words you take less damage from magical attacks while this shield is visible on your character.

Was the second shot on the reaper a fatal blow? I have noticed throughout the game that my killing blows only show the number of remaining HP, say a reaper has left. So, if my first shot did say 217 hp damage on first shot, the second shot my only register 73 hp dmg if that was all the creature had left.

Na I know what your talking about but that wasn't I tried this multiple times. Also tried it on the samurai skeleton thing. takes 5 firebolts for me to kill it with the shield on and only 3 firebolts to kill it without the shield on. The ring on magical sharpness cancels it out mostly but that's only if you do NOT upgrade the shield. The more you upgrade it the worse your magic will get

It doesn't seem to bother Crescent Weapons. If it does it very minor. I noticed a big change with spells but my Crescent Katana seems to do the same damage.

It only affect spells.
It has no effect on weapons upgraded into weapon damage, Large sword of moonlight, Phosporecent pole, etc....
Any items that reduce or increase magic damage you're outputting only affect spells, they do not affect weapons.

Well it does increase the magic defense from 10% to 30% so I would say it it's worth it

It raises the guard break, which means your opponent needs more hits to break your block.

There's never a point to upgrading shields. While it does increase poise, you'll just end up in a bad spot one way or another if you just hide behind your shield for multiple attacks even if you're not guardbroken. You can upgrade regular shields just because regular upgrade stones are so abundantu, but shields requiring colorless demon souls are a newbie resource trap.
If you really really REALLY absolutely MUST have a colorless shield at +5, I'd suggest you savescum by uploading your save to PSN, upgrading the shield to +5, dropping it for a friend, then downloading the backed up save and having your friend drop the shield back for you. That way you keep the souls and get the upgrade.

The other 2 replies are dead wrong. Upgrading this shield will increase your magic defense further, as well as lowering your magical power further as well. It can stack with the Ring of Magical Dullness, I believe. Upgrading normal shields with normal materials isn't that big of a deal, but the shields that can be upgraded with Colorless Demon's Souls, like the Adjucator's Shield, will increase the potency of the buff. So it can be worth it depending on your playstyle and how you want to invest your Colorless Demon's Souls.

No