I have a 1998 base. It wil be up for sale in a couple of weeks. I'm buying my nephews Z3. I've ridden them before (only the 6 cyl 2.8 L model) and can say the Z3 is a nice alternative. It is a more refined ride, but the Boxster is so much more fun I can't even begin to describe it.
The Z3 ergonomics are not as well concieved as a Boxster, but a lot of that also has to do with the fact that a Boxster has fewer widgets to worry about.
A couple of things you will never find on a Boxster include ... a seat that wiggles, stick shift that does not center, stickshift that hits your USB cell hone charger, cup holders that work (unless you buy a Bird armrest or similar), and last but not least ... an engine in the front.
The only reason I am selling my base model is to buy a house. After that, I want to buy an S, later model Boxster base, or a Caymen. Jury is still out on which model to buy.