I interviewed this year with both an Oncure practice and with 21st Century.
Oncure: Basically serves as an administrative/venture capital umbrella for practices countrywide. The group I interviewed with in FL described it as follows: the practice was physician owned and independent for the most part, with Oncure providing administrative support and capital for new technology in exchange for a cut of global revenues (prof + tech). The docs in the group intimated that they still did very well as partners. All in all, it seemed like a reasonable model. My best advice would be to assess your own comfort with that model, and then judge the individual practices on their own merits.
21st Century: Insight comes from first, second, and third hand knowledge, so take it for what it's worth. Practices are nationwide, but high concentration on Florida's West Coast (Sarasota through Naples). 21st Century as a corporate entity also takes steps to ensure that "cutting edge" technology is widely available and in use in their facilities. At the interview, they will tell you that they don't dictate practice principles to their docs, which conflicts with some second and third hand reports, but I'll give them the benefit of the doubt. One of my colleagues who went on a second interview a few years back said that the initial salary offers were quite high (300-400K).
I suppose my main issue with the 21st Century model is that their partnership contracts are highly incentivized based on productivity, which in some offices creates a situation where partners are in direct competition for patients. I learned from several second hand sources that the work days tend to be rather surgical in scope (i.e. 6am-9pm), because failure to do so puts you behind in your productivity.
Without being overly editorial, my bottom line observation with these groups is that they would be good practice opportunities for some, but not for me based on my direct interactions with them. I'd advise anyone who is interested in any of the larger groups to explore them further as the interview season progresses. I didn't mention it above, but one beneficial aspect of being a "corporate" radiation oncologist is a certain amount of practice security that results from being well capitalized.
Feel free to PM if you'd like