Not sure about Good free skins, I am sure there are bad ones though. And its PHP, I have never coded, but I cant think its much more difficult then coding IPB.
vBulletin has been the standard for quite some time now. IPB gained a big following at one point due to its being free, unfortunately they went paid along the way and many either bought the license or moved on to either vBulletin or phpbb. phpbb is a nice piece of software and it's still free but the development is slow and they've had some serious security issues for quite some time now.