One of my biggest issues with OmniPlan is it has no network capabilities or web interface. For me, any project in which only I am involved, I don't need project management software for.
I'm still hoping one day OmniPlan will have a web interface.
There are many web-based solutions out there. The problem with most is they are so cumbersome they inhibit workflow to a great degree. This usually comes from a shotgun approach where the designers try to hit as many audiences as possible. That said, I have found one web-based solution which does a very good job of keeping stuff clean and offer a great ease of use.
http://www.activecollab.com/
Hopefully it wont be long before OmniPlan has a web interface, and something that's as robust yet as clean as activeCollab, but with the Omni touch.