'[...] Apart from the "free" VOLT vs. $$ OTMaster, what about functionality? [...] I'm interested in independent opinions [...]'
Hardly considered to be independent, I will give you my opinion and some info on the functionality of OTM.
OTM is a versatile and very powerful tool and it depends on one's workflow which functionality is particularly useful. I will give just a few examples here.
Glyphs and tables can for instance be added and/or edited. For those who use the AFDKO for adding the OpenType Layout features to fonts generated for instance with FLS, it could be a good alternative because importing the features is a piece of cake (features that are not covered by the character set are simply removed).
Although far from being an expert in this area, at the end of the Robothon conference last Friday I demoed importing GPOS features in an Arabic font and subsequently the numerical editing of the mark positioning in the GPOS/GSUB Viewer.
See also the
OTM web page. Sideways related, perhaps this
Typophile topic is also interesting to read in this context.
By the way, Microsoft generously offers VOLT for free but the Euro 255 for OTM is a real bargain if one only takes the development costs into account.