It's my favourite "normal" US sitcom - for something that follows a very standard format, it's consistently interesting - partly because of the setting, partly because of the scripts, but mostly for the characters (typical "fiasco" plots seem to have added bite when conflicted with the characters individual pretentions, or lack of).
As to the intelligence, it can be hit or miss, and sometimes it's hard to tell whether the scriptwriters are just throwing things in to sound right, or whether they are setting up the characters. For example, at one point Niles (I think) was playing something harsh on the piano, and Frasier asked if it was Mahler, Niles said "god no" and they chortled together about how ghastly his music is. But really, the two characters represent the kind of American intelligentsia who would be great fans. There is a kind of scattered feel to such writing, but it's just me being pickly.