what is benefit of using Antennahouse over Apache FOP?

I have been evaluating all commercial engines for almost 1 year now out of which 6 months I spent working with FOP; the main differences that I see are:

FOP:

• Open source, free
• Written in JAVA, it should work on anything that has Java on it (at least Windows, Linux, Solaris)
• Is still pre-alpha release (last version is 0.95)
• As of today, nobody in the open source community has released anything for almost 2 years!
• Good if you have basic reports; doesn't support the more advanced stuff (like table-layout="auto", retrieving markers in table headers/footers - part of XSL-FO 1.1 and good for subtotals, etc.)
• Some features are buggy (especially around footnotes, breaking tables across pages, etc). These are harder to track and will pop-up later on when you have already written your XSLTs.
• Will consume a lot of memory - this is an issue if you want to run multiple reports in the same time (for example if you have a web site).

All commercial products have the following:

• Technical support (some vendors will even help you troubleshoot performance issues and debug your stylesheets).
• Are Unicode (can handle most scripts like Arabic, Hebrew, Chinese, Russian, Japanese, Thai, Vienamese, etc.)
• Are XSL-FO 1.1 compatible.

Antenna House

• Commercial, $4000-$5000 range (1st CPU)
• Written in C++, cross platform (works at least on Windows and Linux)
• Among unique features: support for MathML, good support for Japanese writing (top to bottom vertical writing)
• Extensions (barcodes, mathml) are provided at extra charge

Ecrion

• Commercial, $1500-$3500 range (any CPU)
• Written in C++, cross platform (works at least on Windows and Linux)
• Among unique features: a good Visual Designer, Silverlight Word and PowerPoint output, PDF forms
• Very fast, extensions are provided standard

RenderX

• Commercial, $3000-$4000 range (1st CPU)
• Written in JAVA
• Among unique features: AFP support, PDF forms

If your reports are simple, you can give it a shot. But my personal opinion is: don't go with FOP if you have the money to spend on a commercial engine. I used to think that being open source is cool because I can change it myself - it is not that easy to change, and my opinion is that is also very risky because you don't know what functionality will be affected. The functionality is actually quite complex, and that is why is not being actively improved anymore.