In youth soccer it was always man defense (in the back 4), but as I got older it became more zone.  I think the general idea, on the coaches end at least, was that as you get better you can handle zone d and stay with runnerst that move around.  My personal opinion is that professionals can handle zone, but even for them, in certain cases (ie, when they play againts extremely dangerous players) man marking is necessary and smarter to employ.  I don't necessarily think that an entire back four (or three) should man mark, but when playing against a game changing midfileder or striker it's best to put your top defender on them and attempt to mark them out of the game.  I've seen too many games where someone like Kaka is given too much space because of zone defense and they just tear the defense up.  If a team has a defender that is good enough, they should be able to prevent this from happening.

I think it depends on who you are playing and what type of soccer you are playing (arena/indoor vs. outdoor).  In indoor - zone is gonna get you scored on.  It's the number 1 mistake I see when we bring new people on the team .  Outdoor I think you need to know your skills - can you outrun the offense? do they have better foot work?  are there any injuries on your defense?  how well is your goalie doing that day?  I spend most of my time on indoor fields lately (easier to field a team consistently week in and week out throughout the year) and we will switch to zone when we are down players (min players is 4, max players on the field is 6).  So if it's a tough week and we only have 4 players show up to a game - time to circle the wagons and keep the other team out.  Longer sends to the other end of the field and keep it out to the wings.

I'm a fan of man to man defense, but your defenders  obviously have to be aware of the other defenders and threatening play makers on the opposing team. If you have a team that plays TEAM defense you'll be set and will not need zone defense, but if you have a team that is concered about themselves and their mark not scoring well then you have a problem on your hands.

I am not against zone but I think it should be used when the time is right against certain teams but not all.  It's always good to have options on defense so if a team can handle adding zone to their game then more power to them.

Im more of a zone defense minded player, usually i only do a man on man when i mark a fast paced foward, but then again when you do that you need good defensive partners. Just my preference i guess, but as we know situation changes and it differs from team to team.