The main problem I've seen with virtualized Active Directory (DC) Domain Controller (DC) computers relates to time synchronization issues. AD is very reliant on good time sync between your DCs so make sure that your hypervisors configured per the manufacturer's specifications to allow the guest VMs to have solid time sync.
Beyond time sync I don't have any bad experiences with virtualized DCs to report. Don't do anything with them that you wouldn't do with physical domain controllers. Be sure that you're not rolling DC VMs back using features like snapshots because you could cause database replication issues (equivalent to restoring an old backup of a physical DC). Don't clone DC VMs (equivalent to disk-imaging physical DCs).
I strongly recommend keeping at least one physical DC around, as well, to echo @MilesErickson's answer. I'd go so far as to say that you need one physical DC in each location where you're hosting server computers to allow those machines to be able to be "cold started" when WAN connectivity is down.