1) Due to my experience - no difference at all. Using on my notebook for 2+ years.
2) http://unix.stackexchange.com/questions/7122/does-lvm-impact-performance
3) LVM (and Veritas) became standard de facto for any serious company. Not only because diskspace management, but also for data management (when you need to move/copy online database from near 100% full diskspace server and so on). In my experience the only danger with LVM are users/administrators actions - such as wrong commands with space resizing and deleting.