I purchased version 6.0b in late 2010. What I did not appreciate at the time is that I had bought a non-universal application with no upgrade path to intel macs (which appeared 4 years previously) that depended on Rosetta (which no longer exists in 10.9, 10.8, or 10.7). The upgrade to 6.7e (as opposed to 6.9 for windows users) involves a hefty upgrade fee. They kept all of this, including the need to pay for an incremental upgrade, well hidden at the time of purchase.
I basically need this only for editing slides in Keynote that contain mathematical expressions.
One thing that would make upgrading worth the additional fee would be Keynote iPad compatibility. Currently there is none. I tried manually installing all the fonts, and converting their non-truetype fonts into truetype first, and then installing these on my iPad using AnyFont.app. Still no joy.
I'm now making all new slides using LaTex. I just typeset the equation, compile a PDF, and then cut and paste. (TexFog is a handy free app that makes this easy).
If you don't need iOS compatibility, and don't mind being chained to the paid upgrade cycles, the software performs well, and the ease of use (which I gave 5 stars) is its strongest point. (You don't need to use LaTex or any other markup language.) It is reasonably feature-rich (I gave 4 stars due to lack of iOS compatibility).
The other rather civilized aspect is that you can make slides using the trial version, and they will still work even after the trial expires, as the application makes its fonts available to the system.