... generally I think that [Grieg]'s a greatly overrated composer who had the happy luck of being able to write good tunes.
No doubt about the melodic gift. But Grieg also commanded an instantly recognizable harmonic vocabulary that (here agreeing with Alan's view) reached its highwater mark in some of the shorter works -- Lyric Pieces and especially the chromatic and modal settings of folk tunes, opp. 66 and 73. Ask Delius if Grieg was overrated... And consider Debussy's reviews of Grieg, which, though not uncritical, indicate that Grieg was an influence Debussy paid attention to in the 1890s. Beyond the short works, anyone who has not listened to the third violin sonata or first string quartet lately may raise their estimation of Grieg by so doing.
That said, I agree, Mark, that many works by the later 19th century Germanic composers I've learned about through this forum have aged better (for me) than Grieg has. (Not to mention that I've enjoyed Winter Legends at least 10 times since the last time I put on Grieg's concerto.)
I do ask myself, though, whether my perspective on the once-radical anti-academic or extra-academic stance of a Grieg, MacDowell, or especially Balakirev and company is determined more by the cultural cycle I live in than it is by the music itself.