I currently own 3 big block Guzzi's (98 V11 EV, 03 Cali EV, 02 Scura), 4 small block 750's (Breva, V7C, 2 Nevadas) and a bunch of late 60's era Stornellos (most in pieces). I'm wanting a Centauro.
There have been a couple of maintenance issues with them:
03 EV had numerous warranty repairs (triple tree, hydro valve, ethanol related fuel pump problem). Since all the problems have been dealt with, the bike runs like a champ.
I'm currently working on the 98 EV. All of the problems were because of lack of proper maintenance by the p/o. (same guy I got the R75 from, and its in as bad a shape as the EV)
Both the Nevada's also suffered from the ethanol problem ( they all have internal fuel pumps and the factory fuel hose wasn't compatible with the higher ethanol content that the gov'ment mandated).
I did a preemptive fix on the Breva (plastic fuel filter was starting to get disfigured) and we have had no problems at all with the V7.
Other than that, I love my Guzzi's. They're fun to ride and easy to work on. Meaning I didn't have to buy ANY special tools for them (unlike my 'modern' BMW's...........

)
I think the main thing with Guzzi's is that they aren't common. And the lemmings out there don't want to buy something that no one else has. Factory support can be trying at times. And they don't have fancy boutique dealerships. In fact, the largest volume Guzzi dealer in the country has a shop/showroom about the size of my garage. No rack upon rack of chinese made clothing and tables full of overpriced trinkets. No espresso bar, no comfy couches. But you can go and scratch Hoffas ears and feed him a Milk Bone. He will love you forever if you do. (Hoffa is the guardian Pit Bull that belongs to owner Dave Richardson)