We use it heavily at work. (daily vendor pricing updates, Lifesaver beta test program downloads, pcAnywhere access to/from home pc, online banking to see if checks cleared, remote backups, print searches, research, immediate company email, contacting vendors via the web, instant messages, and of course - THE GRUMBLE!

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In our case, the cablemodem feed is CHEAPER than dialup. This area of the country has local message units (7 cents for the first minute, 3.5 cents for each additional minute for local calls). Factor that in with a dedicated line(~30/mo), and it would far outweigh the cost of the cablemodem. The extra speed and "always on" are bonuses.
Business cable around here is about $69.99/month. DSL is $49.99/month.
Norton Antivirus is installed on all pc's. We also put an inexpensive router (Linksys BEFSR41 ~$50) on there to serve as a firewall and to share the line with multiple computers in the office. We lovingly call it the "fisher price router".
Since it's just the two of us, it doesn't get abused. If we had multiple employees on their own, I might be concerned about them using the internet instead of working, and might have some sort of "acceptable use policy" for them to sign at the time of hiring. Since we're only 11.5 months young, it'll be a while before we have to worry about this.
Mike, resident computer geek