Personally I'd say don't. My experience over the years has been that when new they are ok. as they get older, even if a factory fit, they leak, mechanisms jam up, many have drain channels which block up and are a sod to clean out. Generally they are bother you can do without.
Having said that our Becky (2012 dynamic eco) has a factory fit, electrically powered, glass roof! I'm running book with Mrs Jock as to when it's going to go "wrong".