Sunglasses for cycling - what features to look for? I need some new sunglasses for cycling - what features will actually be helpful - I've seen ones with vents, polarized lens, rubberised frame, replaceable lens . I just need the best value model that looks good.
Definitely something plastic, as that's going to be both safer and lighter than glass.

I'd look for something that was polarizing, as that will reduce glare, and something that offers UV protection.

You also should be aware that different colour lenses suit different light conditions, so you might need to make a decision based upon when you normally ride. I note that you live in London - I'm in the UK too and given our frequently poor light conditions, for the majority of the year I ride with clear (but high contrast) lenses. 

Here's a colour chart I found a couple of years ago, to give you an idea. 

As for replaceable lenses, they're a great idea but make sure you have an idea of the costs involved first. I got some glasses a couple of years ago and found that the cost of the spare lenses was disproportionate to the cost of the glasses themselves. (But still cheaper than 2 pairs of glasses.)

Another thing would be fit. You don't want them moving around as you cycle - I made the mistake once of buying some nice glasses mail order, when they arrived they were too loose on my head.

Lastly, if you come the the same conclusion as I did about using clear lenses, consider safety glasses such as these. They hit the same quality standards as bike glasses but are a fraction of the price. They are more fragile than cycling glasses but if I go through three pairs a year, its still cheaper.

Aside from that, how much do you want to spend? The pros seem to ride Oakley Radarlocks at the moment but, for what they are, the price is staggering.