I don't personally use the word "sellout", but I have had instances where bands I like move in a different direction and lose me. Metallica is a great example, because while it is common for people to knock them for pursuing mainstream success starting with the Black Album, I just personally didn't like the direction they were heading in (although I wouldn't say the Black Album and all post 80s Metallica is bad by any stretch). This is not because it was more popular, and not because they were experimenting; it is because they deviated from what they were in the beginning, and not in a good way. The thing is, a band is always making a gamble when trying to change their sound; their hardcore fans might like the result, and they might pick up new fans, but they could also end up doing the opposite of both. I like some bands that evolved over time (Deftones, for instance), and others have disappointed me when they tried to change (such as Incubus). To sum it up, change is good, but not inherently, and there's a huge layer of subjectivity thrown in. As for me, popularity just isn't even a factor; I either like it or I don't.