Talk:Rumble/@comment-137.112.171.134-20120122063615/@comment-4560335-20120122215657

Penetration is only good if you stack it.

1 AP improves Flamethrowers damage by 0.3%, Harpoon by 0.285%, and Ult by 0.17% (Instant)/0.11% (DoT). Let's focus on Flamethrower since that's the bread and butter.

Magic Penetration increases damage more the less MR a player has. For instance, if the player has the flat 30, 1 point of MPEN will increase damage by 0.6%. At 60, by 0.39%. At 90, by 0.28%.

Late game it definitely is not a very good item. Normally players are sitting around 80 MR (outside of tanks) depending on raw champ value. Now take 14 MPEN from Runes and 10% from masteries to drop that to 59.

Haunting Guise will improve damage by 16.5% for 1485g (90g per % increase) compared to a Needlessly Large Rod increasing damage to 24% for 1600 (66.5g per % increase). This makes Needlessly Large Rod 35% more cost efficient (Not including HP in Guise. If you reduce the amount Guise "Cost" by how much HP "Cost", that reduces the price to 974, or 59g per % increase. This is certainly something that needs to factor into buying one, but I'm going to mostly focus on the damage aspect).

So how much MR does the opposing enemy need to have for it to outclass Rod? The answer is 20 (AKA reducing your enemies damage to 0) which will improve damage by 24.2%.

So only build guise if either 1) The enemy team builds no MR or 2) You go massive -Mpen with Shoes, Guise, Scepter, and Runes (Which would allow the enemy to build up to 74 MR) or having a MR shredder on your team.

All that said, raging at someone for buying one at any point is pretty stupid. Even if it gets outclassed damage wise, the HP still helps it be an effective item. It's effectiveness v.s. other effectiveness is situation though; What's rage worthy is those that have the "one-build" mindset and don't mix things up depending on what's happening.