I have the RX100 I and I think it's a great take-anywhere camera. I am very impressed with it outdoors in mid-range of its zoom. It rivaled my previous Pentax K5 with DA21mm limited lens in a lot of situations. The Mark III is extremely tempting because the viewfinder and f/2.8 long end are really big improvements in my books. I like available light shooting. It's ok at the wide end on Mk I but then gets really slow quite quickly. They are great video cameras as well. I'm just not sure I want to shell out $400 for the upgrade.
If you're wondering if the RX100's are good, don't hesitate to get one. They are the best of may worlds with quite painless compromises.