A number of studies (e.g., Hession, Rolland..,2009), suggest that low-carb diets do just as well in terms of outcomes while leading to less attrition.
In one paper, a very low calorie diet (VLCD) is described as being around 450 calories per day, but papers all vary. The designation seems to go up to the point of a 500-calorie daily deficit.
One study looked at the lack of predicted weight-loss in low calorie diets and concluded that it wasn't due to greater energy utilization but was rather attributible to lack of patient adherence.
A study by Marzetti and others (2010) examined the benefits of CR on cardiovascular aging.
Hainer and others (2008) provide a good overview of treatment effects on obesity.
Mullins and others (2011) talk about VLCs and ketosis. He argues forcefully that ketones aren't a problem at all for most people on VLCs and that kidney and liver function actually tend to improve.
Reviews of research have generally found that the more closely people monitor themselves (diet, exercise, weight), the better they do.