Currently, Nami has a very low winrate, low enough that a lot of people have stopped playing her. But I think that most of this is due to people playing her wrong, not her being underpowered, and I think that buffing her would not be a good idea---although I wouldn't mind it, since I already have a very high winrate with her. :) Hopefully this guide will be able to shed some light on how to abuse her skills.
Note that Nami requires a HUGE amount of coordination with her ADC to be used to her full potential, definitely more than any other ranged support and perhaps more than any melee support. It's not just that your ADC needs to be good; you and your ADC either need to be in voicechat or need to have played together for long enough that you don't need to talk in order to be very well coordinated.
It can still be viable to pick her in soloqueue because her teamfighting is amazing regardless and her laning is survivable even without coordination, but when Nami and her ADC are well-coordinated in lane, I think she's actually one of the strongest supports in the game.
This guide is intended to be a very brief introduction on how to play Nami, and I'll write a longer guide when I've had a lot more experience playing her (probably around 100 games with her, currently I have about 25).
I've been running flat armor reds, flat armor yellows, and flat magic resist blues every single game on her. Other options for these would be flat AD reds or CDR blues, but only do CDR blues if the enemy botlane has very little or no magic damage (e.g. Nunu or Janna support) and you're planning on getting a very fast null-magic mantle (which is viable, more on this later). For quints, I think that I like scaling mana regen the best, because Nami's mana costs are insane and your power increases dramatically for being able to spam your spells, similar to Sona. Why scaling instead of flat? Because they equalize between levels 5 and 6, and Nami's biggest mana problems are roughly between levels 5 and 9, before she gets her chalice but after she has a few levels in W. Other options are GP10; flat HP; flat armor or MR, to counter a specific kill lane; or MS.
Pretty much the same as what I run on Sona, either 0/13/17 or 0/9/21; when I write a longer Nami guide I'll go into more detail.
Exhaust/Flash or Ignite/Flash if your ADC is taking Exhaust or Heal/Flash. Nothing special about Nami vs Sona/Lulu in terms of summoners.
PASSIVE---It's actually a pretty good passive for early aggression. Use a skill while walking out of well to move quicker, but in general don't cast spells just for the movement speed unless there's a very close chase where it will actually make an immediate difference. Her cooldowns and mana costs are too high to spam spells just for her passive.
Q---This is the strongest part of her kit if you hit it, and completely useless if you miss it. Max this last, but probably get your first point in it at level 1. I think that misuse of Nami's Q is the biggest problem that people are having with her right now. Yes, it's wonderful if you can hit a Q on the enemy ADC because you correctly predicted their movement after getting a lasthit, but there's a huge risk associated with placing a Q because it's so easy to dodge; they can just allin on you if you miss a Q and have at least 12 seconds to do so before it's back up again---so that's roughly a 6-second window for them to engage on you so that your Q won't be up before the fight is over, which is really bad.
Instead, cast a Q whenever the enemy has an optimal place to be standing. This can be after a fight has already been started or when the jungler arrives. If they optimally are running away from you, put a Q so that they can't run away from you without running into it; flash for this if you need to, because a landed Q is pretty much a kill (or you burn their flash, so decide whether this is a worthwhile trade or not). If they optimally are running up to you, put a Q in front of them. If they're standing still in a fight, put a Q below them. Obviously, if they are clearly doing something incorrect, and you can 100% predict their movement, then lead them with your Q (i.e. put a Q where they will be when it pops), but don't expect them to do something incorrect if they haven't already started doing it (e.g. MF runs from a Kogmaw with your E on him; this is probably incorrect, but they've committed to doing it, so lead with your Q).
In general, think of your Q as being like Janna Q---yes, it's great damage if it lands, but the CC is too important to waste, so save it for when you know it's going to be very, very meaningful.
W---This is your main trading skill. It's decent if it hits once, great if it hits twice, and amazing if it hits all three times. Try to go for having it hit twice for the most part. The most common things to do with W are to hit the enemy support and have it bounce to you or to heal your ADC and have it bounce to the enemy ADC.
It's very important to be on the same page as your ADC with this spell so that you can get the bounce off when you're healing him. Of course, sometimes it's not viable to get the bounce off, for example if your ADC is too low to walk forward and trade or if the enemy are both very far from your ADC---not casting this on cooldown is effectively increasing its cooldown so you don't want to wait too long to heal.
Take a point of W at 2 and scale it first.
E---Use this whenever your ADC will get at least 2 hits with it (assuming you have the mana). If they could get 2 hits even without the slow, it's still almost mana-efficient to use it relative to W, and it will likely allow you to get an autoattack in also. Furthermore, it's annoying to be slowed in lane, so you will gain an advantage just by them getting scared of your CC. Do not use E if your ADC is only able to get one auto off, unless you're about to kill them or something; the damage is not worth it.
You can cast this on yourself and hit the enemy support too, but I don't really recommend this because it's not even remotely mana-efficient if you only get a single autoattack off, and it's unusual to get two autoattacks off on the enemy support in quick succession without a larger engage resulting, in which case you don't want to have wasted your E cooldown on the support. Just use normal autoattacks on the enemy support unless you're trying to set up a slow for your ADC to be able to catch up and then also autoattack/use abilities.
If you have the reflexes/synergy with your ADC for it, you should wait to cast E until his first autoattack is physically on the air towards the enemy champion. This will (a) surprise the enemy with a slow + extra damage and (b) guarantee that you don't waste your E due to them backing off before a single AA can hit.
If your jungler is coming or you and your ADC can get a kill yourselves, use your E to set up a Q, but don't allin on the enemy support in a 2v2 unless they're already really low compared to both you and your ADC.
Take a point in E at 3 and scale it after W.
R---It's really slow. Really, really, really slow. Slower than you remember it being. Don't use this to engage a fight because it's too easy to dodge and it's enough CC that it hitting vs it not hitting is likely to change the course of a fight. Instead use it immediately after the fight has been engaged, but without stacking the CC of your ult with any other CC if possible. I like to use E then R then Q, but you can also do E then Q then R; it depends on positioning---if you're close to them, use R first, if you're farther you'll have to use Q first but then your Q will be less likely to land. Like most ults, you should only use it either to save a life or guarantee a kill.
I build Nami basically the same as Sona. It's very important to get an early chalice on her. My opening item progression is usually Faerie Charm --> Red Crystal --> Cloth Armor --> Chalice, but if I think I can go straight to Chalice from Red Crystal then I do that.
(6) Final thoughts.
Nami has enough escape CC that you can get away with a lot less ward coverage than you can with Sona/Lulu. That's not to say don't ward, but you can usually make do with a single ward covering both tri and river paths to lane on purple side or only a tribush ward on blue side.
She feels like a hybrid between Sona and Lulu in lane, where she trades aggressively and sustains with the ever-present threat of hard CC. If you land her Qs she's easily stronger than both of those two supports. Overall, I think she's the most fun support right now.