BMW labels this Z4 an expression of joy. We usually just smile, and the Z4 may well bring a smile to your face, so we'll go along for now. While the retractable top and added features have nudged it a bit closer to grand touring car than sports car it is still clearly aimed at those who enjoy driving.

Both inline sixes are smooth as an America's Cup boat hull right to redline, deliver a sonorous note, are 3.0 liters in capacity and there ends most similarity. The 30i engine is a very light, modern, rev-happy unit that brings 255 horsepower at 6600 rpm and 220 pound-feet of torque at 2600 rpm; it has more than enough power for any road and delivers it in linear fashion, its output rising commensurate with revs. This package is EPA-rated at 18/28 mpg with both the manual and the automatics, numbers we easily met or exceeded.

Although also displacing 3.0 liters, the 35i's is a different engine altogether. It uses two very small turbochargers to boost maximum horsepower to 300 at just 5800 rpm and more noticeably, increase torque by 80 pound-feet to 300 from just 1400 rpm through 5000. The extra muscle gets the 210-pound-heavier 35i to 60 mph a half-second quicker than the 30i and delivers plenty of power for street and track alike. It will wind to 7000 rpm but there's really no point with that abundance of torque, and while it's a superb engine it doesn't offer the emotional happiness the 30i does.

EPA numbers are 18/25 mpg with the manual transmission and 17/24 mpg with the dual-clutch seven-speed. With decent aerodynamic drag numbers, a relatively small frontal area, and an efficient driveline we managed almost 24 mpg in a manual-transmission model over some amusing roads and 38 mpg/72 mph on an 80-mile leg from 4,000 feet elevation down to 700.

The six-speed manuals, both of them, offer soft, progressive clutch take-up for smooth starts whether crawling in traffic or weekend autocrossing. Shift action is light, short and semi-notchy, rather like there's a rubber-edged metal gate hiding under the shift boot. Shifts are quick, clean, and error-free.

The 30i automatic is a conventional six-speed unit and goes about its business exactly as intended; it's not as quick as the manual but costs only in purchase price and not fuel economy.

However, the stronger engine in the 35i gets an optional seven-speed dual-clutch automatic from the M3. Don't let the name confuse you: It does have clutches but they are all controlled by the car, the only coordination required is engaging D and pressing the gas. Around town you will feel like it has a momentary delay between when you press the accelerator from a stop and when the car starts moving; the actions behind this are also why it doesn't creep in gear as much as a conventional automatic.

Pushbutton mode changes allow you to ratchet up the speed and intensity with which it shifts to the point where it is faster than the manual and makes a milliseconds-long burp from the exhaust pipes as it rips through the gears. It's also smart, doing things like dropping gears automatically (rev-matching the downshifts) if you hit the brakes hard to go into a corner, but it will shy away from gear changes mid-corner so it doesn't upset the balance of the car. There is also a launch control mode for ultimate acceleration but read the owner's manual cautions on this before you take the steps and disappear in a wisp of tire haze.

Since it has more power and weight, the 35i gets substantially larger brakes. Brake performance and feel is good across the range, and we had no brake issues at all charging downhill in 100-degree weather in a 30i; those who consider themselves racers may opt for the 35i's bigger parts. The 35i also has wider rear wheels and tires to cope with the added weight and power.

A 35i with Sport Package, dual-clutch gearbox and 19-inch wheels arguably makes the better track car in terms of outright performance, but we found the 30i with Sport Package the sweeter ride on a winding road where the lighter weight is felt, reactions and response seem more linear, and the whole effect is more pure sports car than race car.

Expect the sDrive35is, with its 335 horsepower and 332 pound-feet of torque, and the overboost function that provides temporary doses of 369 pound-feet, to be even more exhilarating when you step into the throttle. This promises to be a very quick roadster, indeed, and will deliver performance levels exceeded by only the most powerful, and expensive, supercars. For those who want the ultimate Z4, the sDrive35is will surely fill the bill.

Even on the standard run-flat tires (no spare) and optional Sport Package the Z4 rides commendably well. Part of this is good suspension tuning, part from the rigid structure it's mounted to, and part from the adaptive damping included in the Sport Package that allows for Normal, Sport, and Sport-plus settings.

Steering is electromechanical but you'd never tell by how well it communicates what the front tires are doing. Unlike many sports cars, in which it seems that heavy steering was a design requirement, the Z4 is light around town, weights up nicely with cornering force and reminds us somewhat of the Honda S2000. It can't really match the surgical detail of the Porsche Boxster, but nothing at this price, short of a Lotus, does.

With a low center of gravity and near-perfect weight distribution with occupants, the Z4's handling is exemplary. You'd need something considerably lighter, more stiffly sprung, and equipped with fatter or stickier tires to make notably faster progress. Like the Mazda RX-8 (a light two-door, four-seat, front-engine, rear-drive coupe), the Z4 is not only nicely balanced and goes where you point it, it does so with little drama and it's relatively easy to find where its limits are.

Putting the top down doesn't change the behavior at all because it's the lightest such assembly in the industry (aluminum panels), changes front/rear balance by only 0.3 percent, and puts the weight of the top closer to the ground. You could argue lowering the top costs some rigidity as the triangulation between windshield, floor, and trunk is gone, but there is no cowl shake and only the inside mirror vibrated a bit on poor road surfaces so characteristics don't change.

The one thing you do have to get used to, depending on whether you're looking at the road or the hood, is what your brain might interpret as a momentary delay between when you turn the wheel and when the car rotates and changes direction. Since you sit so far from the front axle and very near the rear axle, steering input tends to send the hood off to one side before you feel the rear tires join the party. It's a sensation the more driver-forward Boxster and TT don't offer, and it's much more muted in the softer SLK.

Among the competitors, the SLK also offers a folding hardtop, while the Audi TT and Porsche Boxster use folding cloth tops; they're perhaps not as quiet and sealed as a hardtop, and not as easy to see out of, but trunk space doesn't suffer as much when motoring top-down. The TTS has the foul-weather bonus of all-wheel drive and a nicely finished cabin, but not the same balance and precision finesse as the Z4. The Mercedes offers many similar amenities but is less a driver's car and more a small version of the SL luxury convertible. The Porsche Boxster has an even better driving precision and the power of the 35i with the driver's engagement of the 30i, but its tariff can rise even faster than that of the Z4.