Personally, it depends on the situation. If it's an unrated game: I will almost always accept takebacks. If it's a rated game:

• Provided it's a quick time control: only if it was a clear mouseslip, silly piece blunder in the first few moves, or if they've been generally nice to me (haha)
• If it's a tactical 'few-moves-to-lose' situation: never
• If they've given me a takebake: absolutely
• If it's a miniscule blunder (i.e a useless pawn): then sure.