In most cases, you should resign as a sign of respect even before it looks like you're lost (like in an endgame where the guy will gain opposition next move). However, if the guy has an unstoppable mate (or queen and king vs king), just let them get the satisfaction of checkmate.
Is it good sportsmanship NOT to resign?

All things are relative. You are correct in that if the game is lost the opponent should resign and look towards the next game. I think the do not give up attitude goes a bit far in the lower levels. Watching tournament games where players of low skill level move the king around waiting for a draw in poor sportsmanship. Resign and get ready for the next round.
i agree with myself 100 percent. Even years later
Just to offer some perspective, because I'm basically new to chess and I googled to find the answer to this question because I really didn't know the answer. I actually think the most fun thing to do is to checkmate someone, and I hate it when I'm winning and my opponent resigns. I feel like they're just being sour grapes and didn't let me have the fun of checkmating them. But after reading some replies here, I understand that that just not the chess culture or etiquette. Resigning is, apparently, more of a sign of respect. Okay, now I understand that, and I'll resign rather than playing until I'm checkmated. Good to know!