“Well-lit”. California, specifically north Los Angeles County.
I agree with the others that “well-lighted” speaks more to the quality than the quantity of light, although if the room has insufficient light for its purpose it probably isn’t well-lighted 
I’ve spent a lot of time in Florida and Upstate New York (which is where I was born in FWIW.)
I’d say there’s a subtle difference – “well-lighted” suggests aesthetically pleasing light (different colors of bulbs, perhaps, or interesting interplays of illumination and shadow), while “well-lit” simply suggests the appropriate level of light for the intended activity (reading, romantic dining, or whatever).
I grew up in Northeast Ohio, but have lived in Indiana for the last four years.
I’ll go against the tide here. I’ve always been taught that “lit” is acceptable but that “lighted” is preferable. I vaguely remember hearing that “lit” is technically a colloquialism, much like “snuck,” that has managed to enter into the English language but that “lighted” is the ‘correct’ past tense .
Hit submit too soon. I also think “well-lighted” sounds better - it has a nice lilt that “well-lit” doesn’t have - but I’d probably be more likely to actually say “well-lit.”
I’m from Louisiana.
Lit - I guess it’s about the same as being well hung as opposed to well-hanged
“Well lighted” suggests a professional opinion, e.g. from a film professional expressing a technical desirability.
“Well lit” suggests a more generic description of a bright space.
From Maryland, currently living in San Diego.
“Lit”. Sweden.
Well lit. Texas.