Played and finally finished both Double Agent and Conviction.
I found that double agent is a pretty good game if you are willing to forget about the bugs and the tension. Some of the missions are just cool. If they had just worked on this game a little more, I think it would have been better than Chaos Theory.
Conviction has great graphics, an interesting story and palatable gameplay for the average guy, but all the time I played it, I was thinking, "Hey, this feels like Prince of Perisa!", "No, wait, this feels like Assassin's Creed!"
In the end I just came to terms with it, thought of it as "Splinter Cell: Alpha Protocol" and played it. Decent game but definitely not a true Splinter Cell game.