The man, like his three former bandmates, is quite simply a legend. I love his solo work (I'm even a massive fan of Sometime in NYC). Yes it is a bit hit and miss compared to The Beatles catalogue but then whose career wouldn't be when standing next to that!?! They all produced some great work away from the band but suffered from the comparisons to previous glories.
As for Lennon solo, no it wasn't a waste - Imagine, Plastic Ono Band, NYC, Double Fantasy, Rock n Roll (love swamp blues style Sweet Little 16) are all amongst the finest albums ever in my opinion. they stand up just as well as say All Things Must Past, Band on the Run, Ram, Ringo, Tug of War, Living in the Material World and the other great solo albums.
Hell I'll even stick my neck out and say even some of Yoko's stuff on DF, NYC and Milk and Honey is great (Namely: Born in a Prison , Sisters O Sister [both way ahead of their time], O Sanity and Kiss Kiss Kiss).
Outside of music he was perfectly entitled to take 5 years off - he didn't owe fans anything. I also believe his campaigning to make this world a better place was time well spent - might not always have been effective but he never forgot the issues that mattered to him and consequently kept them in the public eye.