I think the definition has been thoroughly covered here by others but I will add that they can be great to get action going in a nitty game. Sure they can steam everyone to the point of breaking the game (if only one player straddles too much and the other players remain nitty...thinking in games where more than just 2X the BB max straddle is allowed)
I disagree that make straddles make it less likely for weaker players to make mistakes, in my experience is causes them to make more as they call straddles more than they should. (they are just going to make those mistakes with players in the straddle pots) If you dont play straddled pots, the fish will mostly give their money away to the most aggressive players in those pots. I agree though that they artificially change the stakes, however knowing that, you can decide whether or not to play in it and adjust your thinking to a bigger game.
Not sure if its the same for everyone, but from my experience the Mississippi straddles will be almost universally button straddles, and will put more money and players in pots than standard UTG only straddles.
For cash game hosts:
If you've got gamblers in your game and they are bored, then consider allowing straddles (in front or Mississippi with Button straddles getting first option, then allow double straddles, and even triple straddles. Double/Triple straddles can make for a maniac game but will keep the button from having too much power as Rhodeman has stated. That will keep the button in check.
(Where I deal they play a 5/25 game, in which there is no SB/BB, only a forced button (Mississippi style) straddle anywhere from $5-$25 in $5 increments. $500 min buy-in, most BI's are 1k+. There are often double and triple straddles. I usually deal a few 2K-$3K PF pots induced stricly from straddles and restraddles that get raised and fought over for meager holdings a night. Knowing that a larger button straddle ($20-25) to start can cause an avalanche of restraddles and crazy action keeps most in check. The game usually runs 3 days straight with most un-busted players only leaving to smoke or go to the bathroom.) While not the same as a Blinds game, it yields some interesting insight into the nature of Mississippi straddles.
If its mostly tight players, but the action is way too dry, I would say allow Mississippi straddles at 2X BB max with no double straddles. With second option going to UTG or just a 2X BB max UTG straddle only. It will put more money in pots and still keep the game sane and not turn away tighter players.
Sorry for the long reply. Drank too much coffee.