If asking a simple question resulted in frowny faces, no doubt my review will be unpopular among the fanboyz. Fortunately, the Omni people addresed this issue in a recent (02/01/2012) blog post:
"Also new and notable! We’re getting started on multi-project support."
This had been requested since 2006.
Version 2.0.3 was unable to open a MS Project MPX file, both using File>Open and dragging the file to OmniPlan. I had to Force Quit OP both times. Merlin opened the file in about 2 seconds.
It did open the MPP file of the same project, and that was a bit quicker than Merlin.
Merlin had a much cleaner view of the project, and this applies to Tasks or Activities, Resources, and Calendar. Merlin offers an additional view (Net Plan).
I tried the "Scale to Fit Project" option only to discover there's no Undo for this.
No problems with ease of use or stability. 4 stars for each of these categories. In terms of features and value I can't rate this higher that 2 stars.
I'll give it another try in version 3. For now, FastTrack Schedule and Merlin are clearly a better choice, and Merlin costs the same as OmniPlan 2.
People with simple PM needs will probably enjoy OmniPlan, but for the same price there are better, more robust alternatives out there.