Melanie, sunnies are awesome. Buy good ones.
Lent a pair of sunnies to a mate last week, paddling down a local river after trout. He lost them.
Anyway, it didn't bother me too much. I bought them in 2004 and they were still perfectly usable. They cost me $290 but had well and truely done their service! I wore them at work every day for 5 years, they took the impact of stone chips while shaping stone, dust from all manner of crap (structural landscaper so plenty of abuse). They were glass, polarised and photochromic (adjust to light intensity from bright to dull).
After about a year they flung off my head, folded themselves up and landed on there end, folding them in half, like, lens to lens. I folded them back out and kept wearing them. The pretty colour had come of the nose piece and i was devastated but the glass lenses had not a sign of damage. Good excuse to by some more for "out and about" and down grade those to the "hack" pair. I have dropped those glasses, lens first, on rocks in the river and not a scratch or mark on them. SInce that first pair i have bought 5 other pairs of the same brand. They are one of the most optically correct lenses you can buy with no peripheral distortion what so ever, which is something that will give you eye strain from long use. As mentioned before, polarised glasses are great for glare reduction. No doubt about it. When you wear sunnies all day, staring at water for fish and forget you're wearing anything at all it is fantastic!!!
These glasses do not come cheap. The brand is Serengetti. Full retail is between $250-400. I used to buy them from a clearance outlet in Melbourne. Cheapest i payed was $120 for polycarb/polarised/photochromic, most expensive is my current pair at $360. The only pair of these glasses that hasn't lasted me more than 6 years is a pair that were 2 years old when my puppy ate them (another set of polycarb lenses), oh, and a pair i left behind somewhere once (d'oh!).
Your eyes are worth looking after and good glasses last as long as you look after them (wouldn't leave your rifle behind, would you???).... doesn't hurt to have a bit of style while you're out and about as well.
Have fun shopping.
Cheers from the sunglasses fanatic.