good (: im. lvl 30 and i still use it, very effective

Josuu is right about that. There is BBCode Guide just a bit under the Start new Guide button. It can help.

As for the guide, it looks quite good. However, I think a Trynda with no defense items is rather stuff for 3v3s or chasing those who lost their herd. In a big teamfight, crowd control brings you death.

You focused on maximum damage, for that, I suppose Greater Quintessence of Critical Damage and same mark would do better, adding up to 33% crit damage. For the penetration, I would take Youmuu's Ghostblade instead of a Phantom Dancer or Last Whisper instead of second Bloodthirster. Maybe even both. While Youmuu helps mostly with squishies, it has many useful stats. LW helps mostly with heavy defenses, making Thornmail much less useful and tanks dead much faster than with two BTs.

The guide has its upsides and downsides and I didn't decide for either +1 or -1. Let it stay this way then. When you make some upgrade or update, let me know to reconsider my vote.

Seems like fair guide, however, it needs some coding.
Read jhoijoi's guide how to make a guide.