I started using OmniFocus initially, following on from using the GTD scripts developed for use with OmniOutliner, the original inspiration for OmniFocus. I was aware of Things when it was launched after OmniFocus, but I held off until certain features were made available. Once those features were there, I started using Things and have been more than happy to use it ever since.
So now the two questions I get asked more than anything are, which do I recommend and which do I use.
The answer, I recommend both but I currently use Things!
Each has it's own advantages and disadvantages, but for me, Things has the right combination of power and flexibility and allows me to task manage without getting in the way.
OmniFocus is a GTD powerhouse and in the right hands, with some investment of time and effort, it can become an incredibly powerful tool.
If your serious about task management and want to get a good feel of the differences between OmniFocus and Things from a users point of view, I'd recommend having a listen to two of my podcasting buddies from the MacRoundtable (David Sparks and Katie Floyd) on the most recent edition of their podcast - Mac Power Users.
In Episode MPU 013 - Task Management Smackdown - they compare notes on both applications.
Just one caveat though, David is a GTD ninja and has lovingly melded OmniFocus to integrate fully with business and personal needs. He's obviously an OmniFocus power user and you really need to listen to the podcast in the context of his vast experience of OmniFocus, it will certainly give you some things to think about but don't expect to reach Davids level of expertise overnght. Katie is probably more like the rest of us (no offence David!) and uses Things on a daily basis to manage tasks.
The level to which David has been able to use OmniFocus probably says a lot about its depth and power, but you do need to put the effort and discipline in to get the most out of it. Things is much simpler and easier to get a rapid return on your efforts. That's not to say Things is not powerful. I use it for organising my daily tasks, my scheduled tasks and my projects. I also use it to capture tasks directly from my email system.
One correction in the podcast is that there is a way of grouping tasks and projects together in Things to give you the 10,000 foot view, and thats "Areas of Responsibility". Check out my screencasts for more information.
I should say though that you need to read the comments on the show notes too, there is some useful info contained within the notes too.