Rare was a brilliant developer back in the early N64 days in my opinion because they had an attention to detail that was matched by no one, and they were being 'mentored' by Nintendo on what was really important; games like Goldeneye and Banjo Kazooie were really large but focused games with an amazing ammount of polish.
As Rare started to take more and more control of their own fate they continued to polish their games but they lost focus; games like Jet Force Gemini, Banjo Twooie and Donkey Kong were good games but far too long.
Even before Rare was sold to Microsoft most of the key project managers had already left and formed companies like Free Radical and Zoonami; Microsoft buying Rare just cause more of the remaining talent to leave and find homes elsewhere. The Rare Microsoft owns today has very little in common with the Rare that was so important in 1998.