I can't really
answer your questions because I haven't got my Fantom-S (yet!) but in my opinion the Fantom-S is a great synth, and with the recent announcement of the upcoming Fantom-X series, the F-S is going for a really great price.
1) The easiest route to expanding your patch set on the F-S is with Roland's SRX series of expansion cards. In fact there is a very nice orchestral expansion card that many consider to be one of the best on any synth, rivalling many orchestral sample CDs.
Roland also recently released a free patch pack for the F-S containing over 1000 of the most popular patches from the older Fantom, XP and JV lines.
Also remember that since the F-S has sampling capabilities, you can expand your sound set with any of the commercially available sample CDs available (e.g. Spectrasonics).
As always, don't just judge a synth on its presets. Many will agree with me that presets (especially on Roland gear) sometimes don't really represent how good a synth can sound.
2) I have that same DVD that you refer to, and I would say that Yes it really is that flexible, probably even more so because that video doesn't even scratch the surface of patch programming.
3) It should work fine as a controller for rackmount gear, can't see any limitations in that area, it's a pretty universal thing.
4) Some people consider the 64-voice polyphony to be a limitation, but that will only really come into play if you do a lot of sequencing on it, playing back multiple tracks at once. For performance, it shouldn't be a problem.
On the surface it appears that it only supports WAV and AIFF files for samples, but there are several programs on the Internet to convert AKAI, Roland, and other sample formats into WAV. Plus Roland is going to be coming out with it's own sample conversion software (as part of the F-X release) that can be used with the Fantom-S as well.
For LOTS of information on the Fantom-S line, visit (one of my favorite web sites) http://www.fantom-s.info/