Same like my build.... Nice... Jungle WW = OP :D

I use Clarity because I've been mid fight and run out of mana so many times, without it I would have died or had to run. But the second reason is my lane partner (friend IRL) is Fiddlesticks, We both time our Clarity's apart to keep each others Mana up so the harassment never stops.

WW with the 6 items posted at 3.1k HP and attacks really quickly. I don't personally like any build with wits ends. The way I play WW has allowed me to 1v3/4. Honestly stepping away from the traditional Warwick set up has drastically increased my play ability with this Champion. My 6th item (Black Cleaver) is never the same depending on who I am facing.

Always this guide was never intended for a jungling Warwick, Lane only.

I agree that increased buff duration is very important for WW, though maybe less important if you don't jungle. The items represent a full burst dps mentality, but WW needs some survivability too.

I am not a fan of clarity. If mana problems are holding you back as laning ww, there are a few solutions: 1) jungle. golem buff will keep you going strong. 2) bring mana/5/lvl yellows instead of attack speed. Attack speed isn't that great without high damage and arpen (of which you have neither at level one) and is found on many items. The mana regen may eliminate your need to bring 1 whole summoner spell for mana. 3) play more conservatively with your mana. Flash or exhaust will serve you far better on WW than clarity, which is next to useless endgame.

Ok, first off, Warwick is primarily a jungler and as such you'd want your summoner spells to be Smite/Ghost or Smite/Flash etc.
As for your runes well they aren't great here is a resource you should read about for runes.
Out of your items I personally think you can do better. Bloodrazor is good and Spirit Visage is ok. You should probably go for Madred's Razor, Wits End, Boots, Sunfire, Banshee's Veil, Upgrade to Madred's bloodrazor. Or something like that.
As for masteries 21/0/9 is good but you should get increased buff duration in the Utility tree (because you're jungling, right?)
Anyway good luck with your guide! ;)