I'm the head of the WAN and data infrastructure for a mid sized enterprise network. I have about thirty sites ranging from half a dozen users (at one site) to a several thousand (four sites).

I use VLAN's everywhere.

VLAN's, properly designed and administered, are one the best tools you have for having granular control of what is happening on your network. Control broadcast/multi-cast traffic, segment different departments, etc. In fact, In almost every site, I have both L3 and L2 VLAN's in place for exactly the reasons mentioned in the last sentance. I also use tagged VLAN's on wireless WAN for point-to-multipoint connectivity and as much as I fought against the introduction of 802.11 equipment being brought into my network, I've now got roaming VLAN's established for the wireless user.

Truthfully, I'd hate to go back to not having them.