There is definitely a list of common problems that occur in these cars if you read these message boards, but you'd find the same thing on an Audi or BMW message board. I say this having owned several BMW's. Do a forum search for words like "considering" and "purchase" and you'll get 100 people asking the same question as you and 10 responses to each, from which the bottom line is generally that the cost of ownership is high but not out of proportion for the type of car, production rates, price, prestige, etc.
This car is not a Honda, it's not maintenance free, but on the other hand, a lot of people here drive the hell out of them with no problems. Maintenance is expensive (around $400-800 every 15k miles for me), but I would guess that's probably not much higher than what you're paying for your Jag.
You'll hear a lot of people say that tires only last 6-10k miles, but I drive pretty aggressively and am well over 30k on my current set. There's little chance your wife will even spend tires as fast as I do.
Get a pre-purchase inspection from a Porsche dealership (very very very important), and spend a couple hours reading posts on this board, and you won't have many (any?) surprises.
Also, there's a guy on here who's name is Randallneighbor who has a great website with some helpful thoughts for people considering Boxsters.
Hope this incoherent post helps...