If I'm playing someone who should know how to play the game, I won't warn them. If I am playing someone new, I might give them a heads up.

On that note, I have played CM Galactus at 1800 points and happily took 6 damage from Ziran at the beginning of the game. I was playing a new player, and maybe I should have warned him that taking 6 damage puts Galactus on his pulse wave clix where he can do a cosmic multi-attack for 9 penetrating damage a turn to everything within 8 squares. Poor guy gave up and conceded the match before I could finish my second turn.

I don't warn people what is coming on my future clix, since they can't see those and legally can't check during a game. However, I do remind them of powers that are currently showing on my clix since the dials and cards can be hard to read from across the table and they may have forgotten what I have.

I will almost always let the opponent know. Exceptions are if the prize is something I *really* want/need or if my opponent is an ### or doesn't follow the same code I do. However, 99% of the time, I'll be courteous, allow take-backs and let my opponent know.

I play a game called Legend of the 5 Rings (L5R) and I've been knocked out of top 4/8/16 placings due to allowing these things. Of course, in those tournaments, we get beers afterwards.

EDIT: I should provide my reasoning for this. I don't do this because I want to beat my opponent at his best or for the sport of the game. I do this because I'm not a dick.