I played around with it last year and I found it a very solid platform. They are really quick to introduce new features. Their calendar and their integrated pomodoro timer are nice, too.
However, for multiple reasons, it’s not for me: no sequential tasks, limited hierarchies, no start dates, no review, etc.
Well, among other things, OmniFocus doesn’t connect to anything if you don’t tell it to. Secondly, if you need to sync, you can set up encrypted sync to your own WebDAV server - in which case Omni doesn’t get see any of your data because it doesn’t go anywhere near them.
As @sprclldr says, it’s quite educational to compare the two.