Have had 6 wins in a row with her so far. Most other Namis I see building AP which is a bad idea.
I build her similiar to Lulu, starting with Chalice and Boots and 2 pots. Generally rush Frozen Heart afterwards then Blackfire Torch/Abyssal Scepter. I would make a Wooglet after but generally the game ends before then. The reason to be building tanky is because she, like Lulu, don't have high ratios and are better off being disruptors and tanky to hold the cap point and assist the team when possible. Both champs have fairly high base stats on their abilities which is all you need for the most part.
Flash and Garrison, max Q first (for cd) then W and E last
Aiming your Q is detrimental between a good Nami and a bad one (besides not going AP like most do). I can't really give too much advice on this besides aiming it in front of the path they are moving or if you know they are going to run. Another time is those who use gap closures and aim it to where they will appear.
Use E on your initiator to keep them from running or yourself when saving someone. Spam W obviously to heal and do some minor easy poking at the same time.
Your ultimate will be used in large fights or when defending top with Garrison.
She's a fairly straightforward champ if you know this and are able to land that Q. She may not replace Lulu but she is still great in the right hands.
Forgot to mention, spam your abilities on your teammates when leaving the fountain to have them catch up, including your Q which does give the passive speed bonus (or yourself if alone).