I was in Ushuaia in February, their Autumn. It snowed and it was wonderful. Wear waterproof shoes that are slip proof. Dress for the cold = hat, gloves, scarves. Dress in layers of clothing as it's warm inside the air conditioned stores but pretty cold outside.
Photo Equipment: It's very picturesque so bring plenty of memory for your camera. That saying, if you run out you should have no problem stocking up.
Miscellaneous: Also remember that in the Southern hemisphere the ozone layer is thinnest here and so wearplenty of sun protection, even when the snow is falling.
Its average winter temp is 1C, while its summer average is 9C, so it is rather moderate in temperature, but has the possibility for extreme swings.
Hiking boots for wet/muddy grounds are advised. Waterproof jackets are too.
Take good walking shoes because the streets are up and down hill.
Also take some clothes to layer, just because it is summer doesn't mean that you will not need some clothes to keep you warm.
Plus you will need a rain repellant jacket.
The weather is very changeable.
Winter in Ushuaia is really cold (-10, -20ºC); summertime is warmer, but temperature rarely exceed 20ºC, the wind is permanent and usually rains, so bring a good waterproof jacket; trekking boots or sport shoes are necessary too.
El invierno en Ushuaia es realmente frío (-10, -20ºC); el verano es más cálido, pero la temperatura raramente excede los 20ºC, el viento es permanente y llueve habitualmente, así que empaquen una buena campera a prueba de agua; botas de trekking o zapatos deportivos son necesarios también.
Miscellaneous: Do not forget the sun block! I am not kidding; sun is quite dangerous at these latitudes.
No olviden el protector solar; no estoy bromeando, el sol es bastante peligroso en estas latitudes.
Luggage and bags: GOLF? Yes!! They have a very nice looking golf course here at the bottom of the world. So pack those golf clubs.