There was a post recently on the OpenWFE developer list asking why Magnolia choose OpenWFE. Here is why:
The decision was made quite some time ago, and it was based on my research and recommendations.
First of all, it was a license issue. I wanted to make sure we can change the license to something other than LGPL (Magnolia's current license) at any time should that prove of advantage. So all projects under GPL/LGPL were off my list. This reduced the list considerably.
Of the remaining projects the exact decision process is lost in time. However, I have looked at the features, the architecture, the perceived success/stability of the project (well, would I have known that John updates his software once a week I might have proposed a different engine :-)).
There were a couple of things at the time that attracted me to OpenWFE, one being that it implements the complete set of workflow patterns, one that its a top project at sourceforge (that makes it easier for me to silence critics), one being that it is cross-platform (that's a big advantage given the fact that there are no homogeneous systems in production).
Finally, I contacted John and found that he is a very likeable fellow (he sits opposite of me at the very moment). It is quite helpful if you can just call someone, or even meet in person, to discuss issues beyond "how to I open a terminal?". So the fact that OpenWFE is a European project and not based in the USA was definitely a big plus for us, because it makes communication very easy.
To sum up: license, features, architecture, success/maturity and support drove my decision.
Others will have other needs, and arrive at other results. Or at the same, but for different reasons.