[gopi]

I have all three versions.

I use the iPad version the most, by far.

I think the iPad and desktop versions have their own unique features that make them better than other versions for specific things. ie: There are things that I prefer to do on the iPad, and things that I prefer to do on the desktop.

That is, there are times when I'm sitting with my iPad and my desktop, and choose which one to do something with based on the UI of OmniFocus on each.

That's not true for the iPhone version. I love having it with me, but you're not missing out on anything unique or surprising. You need to evaluate how much of an impediment having to get out your iPad is. For some people, the extra effort to get out the iPad may make them not add things to their inbox, or not look at their lists often enough.

That's it, though, IMHO. iPhone gives you a very small window into your data with very little effort. The only times I've had my iPhone version open next to another computer were:
a) Laptop didn't have Internet
b) I wanted to look at two windows at once - looking at a to-do list while looking at some other content (rare)