Do online achievement/trophies ruin games?

Ehhh, no. I agree that there have been times when I would repeat portions of games in ways that were really not fun in order to get achievements, but for the most part, achievements/trophies have really added a lot of value to the games I buy and I make the most out of them, getting the most value I can out of them.

Depends. For instance. Multiplayer achievements ruin games, and Call of Duty 4 proves that the MP achievements do nothing for the staying power of said mode.

As far as street fighter 4 is concerned, yes i think so. But in most games to me it really has no bearing.

Yes and No. Some are just plain bad (Achieve 1000 ranked wins) while some are more bearable (Try out all modes). Halo 3's mp achievements are terrible where as COD4's mp challenges are better for that sort of award for performance.

No, they encourage you to see more of the game and try new things you wouldn't have otherwise.

Although I didn't get into Cod4 all that much and I think the game is miles away from truly be modern warfare, I respect the fact that developer omitted any MP related achievements in order not to have the achievement whores ruin the game for everyone. I think game developers need to put a lot of thought to the way the implement their MP achievements. Halo 3 is a prime example of how to not implement MP achievements. Gears 2 did handle that problem by only registering one achievement related action so you won't abuse the system. I also like the way MAG will implement experience points by that if you don't work as a team you will get the lowest amount of points but if you do work as a team and obey your commanders you will get bonus points on top of the regular amount of point. 

For me it depends on how it is implemented.

I loved how they didn't put any MP achievements in COD4 because no one was playing for just the achievements. You'll occasionally find people yelling HELP ME PLOX on Halo and Gears is sometimes unbearable due to achievement hunters.

Original Gears online achievements were insane...ruined the game!

Online achievements don't ruin games, they just make for bad achievements.  Some people won't be able to get the full thousand in those games.  Also, in less popular games, online achievements are almost impossible to get without boosting with a friend.  Try getting the online achievements in Prey or Stranglehold on your own.