Are isolated passed pawns good or bad? I frequently face situations in the late middle game (perhaps only a one minor piece and a rook or two on each side, no queens) where I can force a position (via exchanges) where both players are materially equal but I have an isolated passed pawn on the 4th or 5th rank. I have heard that isolated pawns are bad (because they cannot be supported by other pawns) but also that passed pawns are good. Are isolated passed pawns generally good or bad? Is it good to create them?

I realize the answer probably depends on the position, so guidelines for determining when such pawns are good or bad would make for a great answer.
I'm happy when I have a passed pawn, and my opponent has none. Yet, connected passed pawns are much better than isolated passed pawns.

Here are the raw advantages of a passed pawn:


A passed pawn can no more be exchanged against a pawn, so if it gets exchanged, it will be against something bigger.
If a passed pawn is defended by a non-passed pawn, and the opponent decides to exchange a minor piece against your passed pawn, the defending pawn becomes a passed pawn in turn, keeping your passed pawn advantage, in addition to gaining material. It makes it a little more difficult for the opponent to actually do this.


Still, there are caveats with regards to an isolated passed pawn:


Since your passed pawn can't be defended by another pawn, you'll have to mobilize something bigger to defend it. (leaving less material to act on the issues of the rest of the board) 


But:


If you can have a rook defending your passed pawn from behind, you have one less rook on the rest of the board, but you can easily threaten to gain material by appropriately pushing your pawn.
If your opponent decides to block your pawn with a piece, to prevent you from pushing, well, that's one less opposite piece playing on the rest of the board.
If your opponent decides to exchange your pawn, you end up with a rook on an open file.


In the endgame, connected passed pawns beat the opposite king:


If they are side-by-side, they setup a wall of 4 controlled squares, so can't be threatened by the opposite king.

[fen "8/4k3/8/3PP3/8/8/8/8 w - - 0 1"]
If they are connected,  one row apart, the king has to capture the rear pawn first, effectively leaving the front pawn's promotion square.


All these are limited to what the specifics of every position allow.