I've got one of these (and used it to contribute the SC-8850 demo MIDI recordings for a Sound Canvas module recording project). Here are my thoughts on the module:
For SC-55 MIDIs (ie. most western games, many DOS/X68000/PC-98 titles), the module's compatibility is at least serviceable with the SC-55 map. There will be some patch and sample differences versus a real 55, but in its defence, SC-88s and above/newer aren't 100% accurate with the SC-55 series anyway.
But when we get to SC-88 and SC-88Pro, compatibility is considered poor due to several patches being quite different and (in the case of 88Pro MIDIs that use EFX) the effects render very differently on a 8850 due to a different synth engine. For western titles, though, these modules weren't really all that commonly used IIRC due to things like tracker music and CD audio becoming more prevalent, ergo mitigating this caveat; eastern works (eg. early Windows Touhou games), however, can and will utilise them at times.
Which then brings us to the 8850 native map...I think it's quite rubbish, honestly. Some of the new additions such as the piano are nice, but others such as the stereo strings and the Standard 1 drumkit are beyond atrocious. There may be a royal ton of available patches (1640 including the other three maps), thereby making it an interesting "sketching" module, but I don't consider it a good option for a ROMpler, as it occupies a very odd middle ground between the "home studio" Sound Canvas line (limited editing potential, lower quality samples) and professional JV and XV series (effects engine).
So all in all, I do not advise getting this module unless you have something that really needs one of these. Chances are, most people - and their wallets, considering the prices the 8850 tends to go for - will be better off getting a SC-55 or SC-88Pro instead if MIDI compatibility is important, or a secondhand JV/XV module for a Roland sample box.
CM-64, FB-01, SC-55ST, SC-8850, SD-20