They're my favourite band, based mostly on the five IRS albums, and Mike Mills is one of my musical templates as a bass player. If I didn't have them to "borrow" from I'd have written only about half of the songs we play.
The mid period five albums clearly struck a chord worldwide and have some of their very best songs (Nightswimming, Find the River). I guess they just got bored after years of just playing the same instruments and songs and tried something different.
For me, NAIH is their last great album. Once Bill Berry left though the heart seemed to go out of it. The last five albums are, to me, the five weakest. All have a few good songs on them but that's a poor return compared to the earlier stuff.