If your 6'3 like me I usually hit my head on the top when getting in and out of them. I have to sit pretty uncoumftorably when driving them and theres a horrible blind spot on your back left when the top is up. When the tops up its pretty hard to see behind you and on your sides, but with a car like that who ever puts the top up?
The goods, excelent handeling and the car really responds to you. There is no power steering fluid, it is an electronic steering rack/pump and sounds different when turning the wheel lock to lock. They are very easy cars to work on, great acceceleration on teh 2.5 and the 3.0.
If you really want a good Z4 take my advice since Ive driven them all and worked on the ins and outs of them.
Get a 3.0, sport package, whatever rims you want, get a 6 speed (the 2005 Z4's have a short shift, it is the best gear shift Ive felt in a BMW yet.) make sure you get the sport seats, they are the most comftorable. If they come with speaker upgrades, get them it is the best sound system in a convertable Ive ever heard. You can max the base out so much that it distorts the speakers.
And as the M Z4 goes. I havent driven one yet but I got to drive it out of the wharehouse and from only going in reverse and driving about 300 feet it just feels like it blows the 3.0 out of the water.