To "ride along" with the user, we've used RealVNC for years on Windows XP. (We deploy it with an MSI package and an MST to set a per-Customer password.) This is problematic with Windows Vista, but since our Customers have largely skipped Windows Vista (where we have been, grudgingly, using LogMeIn) we have punted on that issue for now.
We've been using LogMeIn on Windows Vista machines, grudgingly, with success. Unless you pay them, however, you don't get an MSI-based installer. (You can do some rather creative hacks with the "free" edition to get an MSI, but it's not worth the trouble.)
Remote Desktop is enabled via Group Policy on every computer (server, client computer, etc). For support issues that don't require seeing what the user sees, we use that.
I had an interest level in solicited Remote Assistance functionality, but I never did the work to get that going. I'd be interested to hear from people who have, because the more I can use built-in Windows functionality the happier I am.
I can't give you a "hard dollar number", but we'd be hard pressed to service the number of Customers (and raw counts of individual PCs) that we do w/o VPNs and remote control tools.