My friend has just bought a Spider (current shape, used thank god!) so I've had a bit of passenger experience in one.
First the plus points: - Build quality is definately up compared to the GT and older Alfa's, as I found out when I sat in the GT at the dealership expecting it to be on a par with the Spider and the Mito; boy was I disappointed. The ride is exceptionally smooth and comfortable (way nicer than in my Z) although this obviously comes at a cost to handling. That said the handling is still sporty, only it's more of a sledge hammer that needs wrestling than a lithe, sharp tool. There are also lots of toys available, and it's got quite a lot of storage space so it's marginally more practical than my Z. Also, he's got the 2.2 petrol four-pot, which is actually a really good engine. It sounds a lot more exotic than you'd expect and has lots of torque.
Now the bad points: - He's already encountered minor electrical niggles (roof-closed sensor) and there are still some bits of dodgy finish (only very minor though). The roof takes about a week to raise or lower, due to the beautiful but intricate tonneau cover. Most are also only front-drive, although they do feature an electronic trick diff so it's not really to the detriment of handling. Also, there are some slightly "eccentic" qualities to the functionality; for example the USB-in audio can be controlled (skipped, etc.) from the muti-function steering wheel but not from the head unit, even though the buttons have identical functionality in all other respects!
So in summary; still classy and characterful, and better than historic Alfa's but still not on a par with Germanic stuff in quality terms.