NAMI SUPPORT IS OVER POWERED.
When used right. Obviously.

Nami is a high skill champion, uses a ton of mana, and is my favorite support champion! And what I've learned is that if you can land a majority of your Q's (Aqua Prison), you can win lane and possibly the game!

Her passive, Surging Tides, gives bonus movement speed for 1.5 seconds, which is enough for a teammate to catch up to an opposing enemy.

Her Q, Aqua Prison, suspends champions in the air for 1.5 seconds and is a game changer. And if you can land a majority of your bubbles, the enemy ADC and Support will be terrified of you! During early lane phase, I often use it as a poke mechanism. I throw one out every 45 seconds to a minute which gives my ADC multiple chances to make plays or trade with the enemy.

Her W, Ebb and Flow, is my favorite ability of her set. Nami unleashes a stream of water that bounces between allied and enemy champions, it can hit up to three times. If it hits an allied champion, it heals; if it hits an enemy champion, it does damage. Something that is interesting and that I've been playing around with is that The bounce range of Ebb and Flow is larger than the cast range, so sometimes casting Ebb and Flow on yourselves can achieve longer range than casting Ebb and Flow.png Ebb and Flow straight on enemy.

Her E, Tidecaller's Blessing, is SO OVER POWERED. When given to an allied champion, it allows basic attacks to go more damage and the hit target is slowed for 1 second. Before I land an Aqua Prison on an enemy champion, I give Tidecaller's Blessing to my ADC. This I so, if my Aqua Prison lands, more damage is done to the enemy, and after the suspension is over, they are still slowed for quite a reasonable amount of time.

Her R, Tidal Wave, is able to change a team fight; however, it's slow as hell. Nami summons a tidal wave that moves outward in a broad line and deals magic damage to enemies it comes into contact with, briefly knocking them up and then slowing them.

Summoner's Rift: My Recommended Build
(Choose to make up full build - Everything in bold is what I normally build)
Starting: Spellthief's Edge, 4 Health Potions, Sight Ward, and Trinket

Essential: Ruby Sightstone, Ionian Boots of Lucidity/Mobility Boots (if you want to help the rest of the map out), Frost Queen's Claim (Complete this item for maximum gold income)

Offensive: Athene's Unholy Grail, Rabadon's Deathcap, Twin Shadows

Defensive: Zhonya's Hourglass, Mikael's Crucible, Locket of the Iron Solari, Randuin's Omen

Consumables (ALWAYS RECOMMENDED): Health Potion, Mana Potion, Sight Ward, Vision Ward

What Lanes Can Nami Go??

I've done Nami in every lane and JG. The only position I actually refuse to play Nami in is ADC.
Jungle (difficult BUT possible): Nami has huge ganking potential if you can land Aqua Prison.png Aqua Prison.

Top: Nami in top lane is fun, but you have to be careful in how you play. In my opinion it's a farm lane unless the enemy is very aggressive (then it's a "You're Probably Dead Lane" unless you're a high skilled NamiSquare.png Nami and can land your Aqua Prison).

Mid: Nami excels Mid lane. Usually it's mage vs. mage and Nami has a lot of sustain because of Ebb and Flow.

Bot Lane (SUPPORT): Of course this is where NamiSquare.png Nami excels. Land Aqua Prisons, heal your ADC with Ebb and Flow, and when trading is going on between ADCs give your ADC Tidecaller's Blessing for maximum damage.

So, Why Play Nami?
Nami, The Tidecaller, is a fun support, a highly skilled champion, and can dominate lane. NamiSquare.png Nami can stop enemy ganks from being successful, make ally ganks SUPER successful, and has a lot of sustain in any lane (both for her and any in-lane ally).