i never quite paid attention to them

especially not with google analytics and piwik etc around

that said you can just zip 'em and put em somewhere if you ever intend to go through them as an archivist ("in 2006, IE6 had X share... and people were searching for Y")

For personal web sites I don't keep them at all.

For company web sites for which I am resposible for tracking activity and reporting on it, so far the answer is that I keep them forever. Occasionally people do ask if we can come up with stats going back X number of years.

Hard drive size is going up faster than the total size of our server logs, so every year they take up a smaller and smaller percentage of our available space.

when I used my site a lot, I deleted them at regular intervals

It's not a space issue, but rather of relevancy for me. I've done at least two redesigns since I launched and one totally changed the path structures. I mainly use them to diagnose "can't find this file" errors and to see where my traffic is coming from.

I generally find only the last 30 days or so of value, but other than a historical graph I'm not sure why I'd keep them.

I started google analytics a few months back, but the numbers it gives don't jive with the raw logs.

I've not looked into piwik, but will.