Hulu Plus gives you access to a somewhat larger content library and let's you watch Hulu on non-PC devices (like the iPad), but it's still limited primarily to current broadcast shows from its three owners: Disney's ABC, GE's NBC and News Corporation's Fox. In addition, many networks are creating their own free apps for such devices, like ABC, one of the networks Hulu supports.
Hulu Plus for $5 per month is definitely an improvement. Most users are, however, still going to use the free advertisement version of the service since Hulu Plus doesn't offer that much more. On top of that, services like Netflix aren't much more expensive and offer much more content. They're also available on multiple non-PC devices, including the three biggest gaming consoles: Xbox 360, PlayStation 3, and the Wii.