Well we can all agree its been a failure in terms of sales success, but in terms of me personally, I've absolutely loved it. I think its the RPGs that have really done it for me. SSBM and Wind Waker are both fantastic imo, and i've really enjoyed the likes of SMS, F Zero GX, Soul Calibur II, viewtiful Joe 1+2.
I'd only really played Golden Sun and Pokemon on the RPG front before Cube, so its what made it special for me. I fell in love with Skies of Arcadia, and then Tales of Symphonia and Baten Kaitos came to my delight. The 'non-rpg RPG' (as I like to call it) Paper Mario: Thousand Year Door was amazing.
There's been some fantastic games on GC, Metroid Prime and Resident Evil 4 are looked upon as the best of the generation by many.
There's also Super Paper Mario and Baten Kaitos Origins to look forward to, and there's many games out I intend to get that I haven't.
So to sum up, yes it was a failure in success*, but no, not a failure for what's important, games.
*And even saying that, Nintendo still made a profit from the GC