critplank is only viable if the other team doesn't immediately focus you. In higher rank, you'll get owned very fast.
Anywhere else is probably fine. Of course they'll eventually buy armor and make you useless again. --- Don't be that guy. That guy just wants to be me. A plan without action is a dream, action without a plan is a nightmare.
You should build critical with cooldowns - I really like to get Glacial Shroud for armor and anti-DPS too. --- The CHA King ~ Garen, TF, Anivia, Ashe, Vlad, Kayle, Nid, Corki, Ezreal, Fiddlesticks, Xin, WW, Annie, Jax, Taric, Kat, AP Yi, Trynd, Poppy, Teemo, 'n Tristana.
In my bracket (I'm halfway to level 21) Critplank usually means you are not on my team and you are legendary with 25 kills 32 minutes into the game, so I'll say its viable for solo queue. --- Uh, Centriole AND Eon_Latias were setbacks! Okay, I never let WoW boards down before. - Left4Shaman