I remember the first time I ever played Splinter Cell on my PS2, and I was blown away by the graphics and gripping gameplay. It was absolutely superb!
Whilst the others may have been a bit too similar, they consistently moved the series forwards and improved it.
Sadly, Double Agent is quite poor. For a start the light meter has gone, so you can't tell how well you can be seen. The inbetween missions bits in the mercenary base are complete tosh and very UNenjoyable, especially as they all have stupid time limits! The whole idea that a man like Sam Fisher would even contemplate switching sides is hard to believe (especially for those who have played all the previous Splinter Cell games).
In summary, I would only recommend this game if it were between £15 and £20, it is NOT worth £50! If you have a 60GB PS3, get the older PS2 versions, they are a lot better! Or if you have a PSP, I would recommend that version, it is top notch!