I have to say, I think that Chuck finally hit it's stride around the 5th episode, and now has become one out of only about three new shows that have made the cut and have been added to my regular viewing schedule.
What improved: I think what's most notably gotten better is that the writers put the "Chuck doesn't trust Sarah and Casey" storyline on the back burner, which was definitely starting to get tiresome after the first four episodes. Instead, they've been developing the "repercussions of the double life" theme; that becoming a spy is not as cool as it may have seemed, as it is constantly interfering with Chuck's romantic and family life, as well as whatever plans he may have had for his future. Another corrected error is finding the right balance between the spy missions, the Buy More and the home scenes, with just the right amount of each and the occasional hilarious intersections. In earlier episodes they made the mistake of overemphasizing the spy scenes, which often turned out to be less amusing then the Buy More and home scenes. Lastly, they've toned down BFF Morgan to the point where I've even warmed up to his character.
Chuck is still not without its flaws - I find it hilarious that every bad guy or love interest he encounters happens to pop up in the same strip center as the Buy More, or wherever else Chuck randomly happens to be while accomplishing the mundane routine of the non spy aspect of his life (criminal masterminds always seem to materialize when he's out to dinner, at an arcade with Morgan, etc.). Furthermore, I am still plagued by why Chuck is so incredibly useful to the CIA and NSA despite the fact that he is not - to use a tech term - "live updating" with current intel - though I guess they have broached this in more recent episodes by suggesting that the government eventually wants to kill him when they get the next intersect (though honestly, I feel like Chuck wouldn't be useful after a day, as opposed to weeks, with the speed that intel would change). However, Chuck has established itself as such a goofy show that I've decided to not be too bothered by these small details.
Furthermore, Josh Schwartz continues to brilliantly incorporate all kinds of pop cultural references into Chuck that just make me smile. Some of my favorites: the clear homage (some say parody, I say homage) to the Countdown episode from the first season of the O.C. - complete with identical dialogue and musical selection "Dice" - at the end of the Sandworm episode, the naming of the shipping heir as Stavros Demitrios (eerily similar to shipping heir and onetime fling of Paris Hilton, Lindsay Lohan and an Olsen twin, Stavros Niarchos III) in the Imported Salami episode, the parody of Mr. and Mrs. Smith in the Buy More shootout during the Nemesis episode and the perpetual references to geek favorites including Bond films, Star Wars, Dune and War Games. He also was even brave enough to use Britney Spears' "Toxic" as background music in one episode, which was oddly fitting.
If you gave up Chuck after the first few episodes, I highly suggest that you reconsider (especially since it's now been renewed for next season and will be returning in the fall).