I recently had some similar questions as I was considering buying regarding the maintenance and reliability of them.
pinheadh78 had some good insight and also service advice, his second post (7/23 5:08) in this thread has lots of good info:
Considering 530, Service & Ability Expectations
Beyond what was said there, all the issues seem to be the same throughout the years, but the early ones came with 12" tires, and it seems those tend to wear fairly fast, however that may have been with the bigger displacement versions. Most feel the 250 was underpowered for such a heavy vehicle.
There are reports of some Piaggio dealers no longer servicing MP3s after they are 10 years old.
I get the feeling if you're a willing decent DIY mechanic, or have a direct source to one, they are reasonable to keep going, however due to the rarity and complexity, either a dealer or similarly-priced shop would make an MP3 expensive to keep going vs a more reliable, less complex bike. It doesn't sound like they are terribly unreliable, just not super reliable either. The added wheel, suspension and tilt lock add quite a bit of complexity to a standard scoot.
Good luck with whatever you decide.