I would say I generally prefer the music more as the series progresses. It's not a linear progression, just a general impression of liking the music more as the show progresses.
Background Music -- I find some of it too sappy for my tastes. In the later seasons, a lot of the background music over the exterior establishing shots is more rock oriented, which I like.
Cover Tunes -- Like some, dislike some. Some I like are "My Generation (Danny with Jesse & The Rippers), "My Girl" (Danny with Jesse & The Rippers), "Do Wah Diddy" (Jesse & The Rippers & Michelle) "Rock A Hula" (Jesse & The Taste of Polynesia band), "Shout" (Feedback), and "Teddy Bear" (Jesse, Joey & Danny).
Original tunes -- My favourite songs on the show are original tunes, though I don't like all the original songs on the show. My two most favourites are "Flesh To Love Affair" (from Rock The Cradle) and "A Little More Love" (from The King And I). Other favourites are those from Claire and Present Danger, and "April Girls" (from Making Out is Hard To Do). Don't like Forever as much as I used to, and really don't like "Michelle's Smiling" (from Baby Love).
Guest Singers/musicians -- Like Tommy Page and Little Richard. Beach Boys are ok in Beach Boys Bingo (though I'm not a big fan of the episode), but I don't care for either of the subsequent beach boy appearances. Don't like Frankie Valli.
The Advertising Jingles -- Generally like these more than I used to, but I like other music from the show more.