Double Agent was an enjoyable experience for people that played the first 3. It really took on a lot of changes that Chaos Theory made to the series (Chaos Theory is easily the best entry in the series and always will be) and added a few goodies of their own.
Personally, though, if you're going to play Double Agent, I highly recommend playing the original Xbox version. The campaign was actually drastically different from the 360 version and had a much more extensive campaign with more levels and a more fleshed-out storyline. The 360 version was by no means bad, but there were parts of the story that felt under-developed compared to the Xbox version.
And just to provide my credentials, Conviction's was a good game, but the developers streamlined it and made it way too fast-paced and easy to play. Half the fun of Splinter Cell was getting yourself through levels without being spotted and having to control your movement and actions. Moving too fast or sitting in a spot with too much light would get players caught. Tools like Sticky Shockers and EMP on the pistol to shoot out lights were the best part of the series, not to mention the fact that Sam only used 2 weapons (Five-SeveN and the SC20k) and didn't have an unnecessary amount of useless weapons.
I miss the old days of Pacifist Ghost runs through the originals... May have to go back and do that again. Been wanting to play SC1 again anyway.
Fear not, my brothers! The next generation of gaming is upon us! Be glad! A reward for all your toil and all your sacrifices is upon us.
There are those who said this day would never come. What have they to say now?
(Yes, I ripped off a quote from Halo 2 and modified it to my own purposes. Sue me.)