They're somewhat pleasant, or at least neutral. It takes work for me to remember difficult or annoying memories, and this is about the opposite of effort: they pop into my head, one replaced by another, unbidden and without apparent connection or emotional charge, like my mind is flipping through someone else's snapshots. The ones I just mentioned took up less time than it took to type this sentence. Often they're mundane and prosaic places no one would go to visit, but sometimes they're not.
I can tell I'm bored when they start to appear. They usually pop up when I'm doing something dull, like data entry at the computer.
I guess it's my mind taking quick vacations while I'm doing something else.