My opinion varies per game series. Some examples:

(+ == improved, - == declined, = == steady, no improvement or decline)

= Baldur's Gate/NWN series - I think this has aged rather well, except NWN2's interface leaves much to be desired. I liked NWN1's interface and handling very well. The Baldur's Gate games based on the infinity engine were all fun, the ones I played at least, but it is a bit painful to go back to them now because the interface is a bit old and clunky these days.

+ Dark Forces up to Jedi Academy - Love em. All of em. They aren't designed to be anything more than shooty goodness with a bit of story attached, and they do this very well. If anything I think they became more fun as time passed and they adapted them to the newer game engines.

- TW series - I think it was great up to M2TW. STW is an immortal classic for me, MTW was ok, I absolutely loved Rome. M2TW however was a poor investment in my view, as I do not like where CA has been taking their games, paying lip service to modding and all. Thus I'd say they're going downhill.

- Fallout - There is no such thing as Fallout 3. *closes eyes and covers ears and chants to self*

----- Need For Speed - Need For Rice. Need 4 Spinnahz. The series peaked at NFS5: Porsche Unleashed and has been a harsh, steep downhill slide ever since.

= Doom and Quake - I think these have aged well. In retrospect, these games aren't really anything more than tech demos, including the original games. id Software doesn't really make games so much as they make game engines and frameworks to sell to other companies. I've enjoyed all of the Doom and Quake series to date.

Nothing else comes to mind as of now, so I'll leave it at this.