In reply to BusyLizzie:
They're marketed as having two uses- to improve endurance and aid recovery. They say they do this through acting on the blood supply.
I'd say it's a popular opinion that they do indeed aid recovery, but don't do too much to improve endurance, if at all.
However, as another commenter said there's not a lot of conclusive evidence either way- the jury is out so to speak.
I don't think I know of anyone who says they are detrimental, however. My only bad experience was wearing compression tights, where I found that they caused me to chafe. You won't get that from socks! A lot of top endurance runners use them- and though that's not conclusive it's a sign they're probably not at all bad.
I think I'm on fairly solid ground when I state that they're not marketed as 'muscular support'. At least two commenters have claimed they are doing this (eg the impact on the achey knee, and the guy who says he switches round to ensure he doesn't adapt to 'need' their support). To decide if they act as muscular support, consider what actual muscular support devices look like... ie much more solid strapping, velcro, and so on.