I've always used an external router hooked up to my ISP's modem/router, because I don't trust their security. In fact, I had a conversation with AT&T once several years ago. I was having trouble with my DSL Internet. AT&T sent a tech out. Our conversation went something like this:
"You know your TOS doesn't allow you to have a router connected to our modem, right?"
"Yes, but I don't trust your security measures."
"But you aren't allowed to have a router connected to our modem"
"Fine. Disconnect your modem and take it back, and I'll get cable. There's no way I'm hooking my computer directly to your modem without a firewall between them."
That pretty much ended that conversation. The modems and routers they use now all have basic firewalls built into them, but everyone I've looked at either had a crap firewall or crap wifi, or both, so I still use an external router of my own. Right now I'm using an Asus router running dd-wrt.