Lipodystrophy- changes in fat composition as a result of injections, especially insulin injections- happens in two major types.
Lipohypertrophy occurs to at least a mild extent in the majority of people who have used insulin for five or more years; it tends to make itself known by a puffiness in the area(s) used most often for injection.
Lipoatrophy, where the fat from an area disappears, is much rarer. I don't think anybody's done any good studies on lipoatrophy's incidence or prevalence, but my sense is that it occurs in 1-2% of long time insulin users.
Both types of lipodystrophy can change the insulin absorption. In most cases, it just makes it less consistant; in extreme cases it may prevent insulin absorption in those areas entirely. Recovery in a specific area may or may not happen after changing where the insulin is injected or infused, changing the insulin delivery method (from pump to shots or the other way around), or changing insulins.
Personally, I have so far experienced very mild lipohypertrophy on my thighs and that's it as far as visible lipodystrophy is concerned. However, I have gained a significant amount of fat because of being unable to walk nearly as much since my foot injuries in June, and I thought I'd try the Quick Sets again. Two out of two kinked, and this time I can't blame a lack of fat... I'm gonna blame scarring and/or abnormal density of fat tissue.