For 2006, the BMW Z4 enters its fourth year of production and the automotive equivalent of middle age. Thanks to a number of significant changes, however, this year's Z4 remains fresh and will likely appeal to a wide variety of buyers. And yes, that even includes those consumers encountering midlife crises of their own. Given BMW's reputation for building fun-to-drive cars, it should come as little surprise that this upscale car can be quite entertaining. This year, the Z4's range has been expanded to include a coupe. The coupe's fixed roof provides extra body rigidity for improved handling and, to most eyes, more attractive styling. When viewed in profile, the coupe's sloping roof line seems to hark back to the classic GT coupes of the '50s and '60s and does much to clean up the roadster's somewhat awkward proportions.

Also new are two new inline six-cylinder engines. On the base 3.0i trim level that replaces last year's 2.5i, there's a 3.0-liter capable of 215 horsepower and 185 pound-feet of torque. Not only is this output significantly better than the 2005 Z4 2.5i's 184 hp and 175 lb-ft of torque, it's almost at the level of the 2005 3.0i's 225 hp and 214 lb-ft.

The 2006 replacement for the old 3.0i is the 3.0si. This car has a high-output version of the 3.0i's engine that's good for 255 hp and 220 lb-ft of torque. These engines feature many new technologies, including the latest version of BMW's Valvetronic variable valve timing system. In addition to the extra power, the 2006 Z4's available engines also rev higher, have a flatter torque curve and are slightly more fuel efficient than the ones they replace. BMW has also introduced a newly optional six-speed automatic transmission.

The BMW Z4 is fun to drive and reasonably affordable as long as one doesn't select many options. And thanks to this year's changes, it's the best yet. Shoppers should certainly do plenty of research before making a buying decision, however, as there are plenty of excellent cars available in the upscale roadster and coupe segments. Others to consider include the cheaper Honda S2000 and Nissan 350Z, the V8-powered Chevrolet Corvette, the prestigious Mercedes-Benz SLK and the Porsche Boxster.

The 2006 BMW Z4 is available as a two-seat roadster or coupe. The roadster has a traditional, manually operated convertible soft top with a defroster-equipped glass rear window. Two trim levels, 3.0i and 3.0si, are available. The 3.0i (roadster only) has features like 17-inch alloy wheels, rain-sensing wipers with heated wiper jets, heated outside mirrors, power mirrors and windows, keyless entry, manually operated seats, vinyl upholstery, a tilt-telescoping steering wheel and a CD player. The 3.0si version adds 18-inch alloy wheels, leather upholstery, a center armrest, aluminum interior trim, a premium audio system, a trip computer and automatic climate control. Most of these features are available on the 3.0i as options. An optional Premium Package includes HID headlights, a one-touch power operation feature for the convertible's top, auto-dimming mirrors, power seats, memory for the driver seat and Bluetooth connectivity. Other options for the BMW Z4 include higher-quality leather upholstery, a navigation system, sport seats and a Sport Package that includes upgraded wheels and tires, BMW's Dynamic Driving Control feature and a sport-tuned suspension.

Two engines are available for the rear-drive Z4. The 3.0i has a 3.0-liter inline six-cylinder engine capable of 215 horsepower and 185 lb-ft of torque. The 3.0si has a high-output version that's good for 255 hp and 220 lb-ft of torque. Each trim comes standard with a six-speed manual transmission. A six-speed automatic with BMW's Steptronic sequential shift control is available as an option. According to BMW, a manual-equipped 3.0si can achieve 60 mph in 5.6 seconds. For both engines, fuel economy rates 20 mpg city, 30 mpg highway with the manual gearbox and 21/29 with the automatic.

Every model comes with four-wheel antilock disc brakes and a sophisticated stability control system. Passive safety features include rollover protection, side airbags, active knee protection and child-seat anchor points for the passenger seat. In NHTSA crash tests, the BMW Z4 roadster earned four out of five stars for its performance in frontal-impact occupant protection. It has a three-star rating for side-impact safety.

In nearly all performance aspects, the Z4 shines. Acceleration, braking and handling are all excellent. The Z4 is in its element on lightly trafficked roads when one can push the car toward its limits and enjoy the engine's sonorous exhaust note. From an everyday standpoint, the Z4 is still agreeable. Wind and road noise on the roadster can be bothersome at highway speeds, however, and some drivers might find the ride quality produced by the optional Sport Package's suspension to be a bit too stiff.

The Z4's simple cockpit features a sweeping dash with a clean set of analog gauges, bolstered sport seats and little else to distract you from the road. Plenty of metallic trim comes standard, but wood trim is available for those who want to dress up their Z4 a bit. Both the manual and power-operated convertible tops are easy to use. The roadster's 9-cubic-foot capacity barely diminishes when the top is lowered. For comparison, the Mercedes SLK offers 10 cubic feet of cargo space with its hardtop up but considerably less in top-down configuration.