Partial ownership: are there any pitfalls to owning the plane through a shell company?
The main thing to do is to realize why you're creating the corporation.  We created a 501(c)7 (in the US) corporation which is a tax-exempt organization and falls under "social club."  We did this to limit our personal liability.  However, if you are creating the corporation to limit personal liability, you have to make sure to operate like a corporation - call each other "co-owners" instead of "partners" (there are legal ramifications to this, not just semantics).  People around the airport should know it's a co-owned plane by a corporation, not that it's "your plane."  Basically if you want the liability protection of a corporation, you must act like one.

Great book to read regarding the legal side of things is "Practical Aviation Law" by J. Scott Hamilton

Depending on how you set it up (we did a 501(c)7 tax exempt, and yes, you still have to file a tax return), how you use the airplane matters too.  For example, we can't earn money using the airplane (conducting flight instruction to non-co-owners, for example), due to the 501(c)7 status.  Under other corporate structures you can.

Finally, I guess to actually answer your questions: There are some pitfalls, but I think they can be avoided.  Most group ownership things fail because of lack of communication and documentation, forming a corporation kind of forces you to spell out things explicitly and then all involved have no excuse for not knowing "the rules." (You have to write up bylaws and have at least one annual meeting, which forces the communication aspect!)