I've always enjoyed McCartney I. I just don't feel the occasion to play it as often as his better works. I consider it to be a light and airy confection, as opposed to a "meal". I believe Paul intended it as such. For me, it was his "this is who I am at home" album. Just another means for him to step out of the Beatles as an individual, and notable by its completely ANTI-political tone, which is how John had decided to separate himself. Tossed off and imperfect, even "half-assed", I think it accomplished its purpose: it got Paul a vehicle out of the Beatles.
Paul wasn't in the mindset at the time to do a "produced" album, with months and months of work involved. He needed something tangible out there quick that said "Paul McCartney" on it. As an artistic work it hasn't much merit, except "it's the album 'Maybe I'm Amazed' is on". Still, Paul has made it clear throughout the entirety of his career that he wants to be able to do both polished works (Sgt. Pepper, Abbey Road, Ram, Band on the Run, etc) and stuff he whipped together in his basement (McCartney, McCartney II, first 2 Fireman albums, etc). And, he wants to do either as the mood strikes him! As fans this can drive us nuts, but there is a certain integrity to it.