Learn something new every day. I've always used Helvetica on all my web pages at the behest of my design co-worker. He's a print designer so it makes sense now.
I hope you do test how it looks on windows? I've worked with a webdesigner that often chose fonts that worked perfectly on his mac, but looked really ugly on all windows browsers, since the fonts were unavailable and the substitutes had noticable style/width differences.
Well, it really depends on how the digitization of Helvetica that any particular user has installed is hinted. The Helvetica that comes with MacOS is quite nicely hinted, and will make for a pleasant read at most sizes.
On Windows, however, Helvetica isn't (at least from my experience) as nicely hinted, or at least doesn't play quite as well with Microsoft ClearType. And that's if it's installed, which is less likely on Windows. In that case, if your font stack specifies a fallback to Arial, which is ClearType's best friend, you'll probably be fine.
Alternatively, @font-face is supported by just about everything at this point, opening up the opportunity to use much better fonts, like open sans
(also, helvetica is a print font. It doesn't work for body copy at the sizes commonly used for screen text.)