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RX100 works pretty well for long exposures, but I'd suggest a OM-D with its very cool real time preview mode (you can watch the image sort of "develop" before your eyes) if you do a ton of LE work, just makes things much easier to judge exposures.
RX100 (with a little home made button pusher) works pretty well using my ND110 10 stop filter though. Just as its not the best camera around for high ISO, its not the best at longer exposure times either, but it stays pretty clean at base ISO (125)
Its not something I'd set out to buy for LE work, but its nice to have a true compact that can give you some LE type shots.
Its not its biggest strength, far from it, but if your wanting to take just a small yet decent camera with you on a vacation or business trip etc, and may just want to grab a few LE shots on occasion, it should serve you well.
Thats what I like about the camera, its small enough to take with me, simple enough for family members to use (typical controls, face detect AF etc) and when I want something more than just a typical P&S or iPhone type image, I can bring a couple of filters, a little tripod etc and make some real photographs that also look quite nice.
Sure beats dragging the whole DSLR rig with my when on vacation just to use a handful of times for some LE type shots. Not "as good" but I'm sure we've all dragged some big/heavy camera gear with us on family vacations etc only to hardly touch it and worry about it in the hotel room etc rest of the trip.