I first used OmniGraffle 2.x (I think that's the right version) because it was bundled with my 12" PowerBook G4. I recently started working on a project that involves a whole lot of diagramming (information architecture for a large web site) so I looked at 4.0's regular and pro versions and went for the pro version, and I'm very happy I did.
Here's what I use from the Pro feature set frequently:
- Master canvases, to share common background layer stuff across multiple canvases in a single document
- Multiple page documents (canvases), to group related diagrams into a single file
- Multiple editing windows, to see other canvases from the same document at the same time. This wouldn't really be useful if I weren't using mutliple canvases in the same document since I'd just have multiple documents open at once, but the reuse from master pages & the conceptual grouping of multiple canvases makes multiple editing windows for the same doc a necessity.
- Manual guides. This is like the Photoshop guides feature (or really any other drawing program's manual guides feature) and helps me make sure my drawings look consistent and orderly, to prevent readers immediately bleeding from the eyes.
- Drawing styles. Yeah I could manually copy styles from object to object but this is faster and again keeps my drawings from inducing ocular distress.
- Mouseless editing is sorta nice, I just have to keep reminding myself I can use it.
I haven't used the other stuff much; I want to try and use the inter-canvas hyperlinking with the HTML or PDF export because I hear that it works as Visio does, preserving links in the output so you can make a quick and dirty lo-fi HTML prototype that folks can use outside of OmniGraffle.
Oh, and I did use the Visio XML import, which helped me rescue an rather precious legacy document from years and years back that I thought I had lost. The line routing wasn't exactly as I had left it but the content was intact so it was no big deal to fix it.
Anyway, they're very fair about upgrade pricing (it's the same as the price difference between std and pro) so my advice is to just pull the trigger and get the standard version. If you find that you wish you had the pro version you can do that easily later. There's hardly any downside with that plan, vs. spending an extra ~$70 on the Pro version for features you might not want, or wasting time trying to do a thorough pre-sale evaluation.