Only the online Cambridge dictionary marks the verb “to expatiate” as ‘formal disapproving’. Nowhere else could I find the reference to a ‘disapproving’ connotation, although all the online dictionaries agree upon its formality. Do you deem it to be used to criticize the act of “speak[ing] or writ[ing] about something in great detail or for a long time” or can it be simply used as a synonym of “talking at length”?
I looked up the word on Wordnik, which, in addition to showing several different meanings of the word, shows a list of sample usages along the right-hand side of the webpage. (The website starts with 10, but you can view even more with the click of a mouse.)
Neither the plethora of definitions on the left nor the example usages down the right seem to indicate an always-negative connotation to the word. While some seem to suggest that the expatiated diatribe would be unwelcome (please, spare me!), not all of them do.
It seems like a good choice of words when you want to express a long rambling, but doesn't seem to always connote "disapproving."