I've got a Mac Mini, as you can see under my name. It's plenty fast and I haven't had any hiccups with it yet. As others said, it's a standalone machine; I hooked mine up to my old keyboard and monitor. I personally use it for my main machine, as I'm not doing anything heavy (graphics editing, music production, or anything like that).
As others have said, I think the person on the phone was recommending a mini because it's the cheapest of the Mac desktop line, assuming you have a mouse, keyboard, and monitor already. I bought it specifically because I was switching from being a PC-only user, and didn't want to sink a lot of funds if I didn't like the experience, which is why the clerk may have been recommending it. I fact, the Mini is Apple's "introductory" machine, no?