Pros:

Amazing harass in lane
Almost impossible to gank
Deals heavy damage
Can take heavy damage if built properly
Allows you to be an absolute *******
He is Teemo

Cons:

In teamfights you WILL be focused
Sometimes hard to deal with caster champions at later levels
Hard to position properly

Your goal in lane is not to take the top tower, it is to farm and harass the enemy top champion. The hardest thing on teemo is learning to get basic attacks frequently on the other champion while still getting all your last hits (I CANNOT EMPHASISE ENOUGH HOW IMPORTANT LAST HITTING IS) If you don't get kills early (using that elixer and your own swagger) the farm will be what matters coming into mid and late game and you do not want to be lacking there or your team will be like oh noob teemo sucks why would you pick him. Or if you do well you could RUB THAT **** IN THEIR FACES AND REMIND THEM WHO IS GOD OF SUMMONER'S RIFT.

To conclude this guide I'll sum everything up. The key to top teemo is to first have the proper runes and masteries, start with the right items, ALWAYS get toxic shot if you can, shroom the river at 6 to avoid ganks (before that use the ward you buy at the beginning). Harass the other top champion heavily and kill them if possible but still focus on farming. Your item sequence will be the start listed above (elixer, ward, health pot) 3 dorans blades and boots, then blade of the ruined king, guardian angel, and the situational items after. You WILL become a split pushing god also, put yourself to use. In lane only use your blinding dart to block basic attacks or finish off a kill, never use it for harass unless you have the mana to spare (you usually wont if you are shrooming everywhere). This guide was inspired by rainman's competitive teemo guide but I changed it up quite a bit to make teemo viable in all theatres of play. And in the department of summoner spells I take flash and ignite because ignite helps secure kills both early and late, and flash helps me escape when I inevitably get overconfident :3 (you can also take barrier instead of flash, that **** is op)