Since 1996, Mercedes-Benz has sold over a half million SLKs, making it best-selling luxury roadster in the world, bar none. To keep the competition at bay, Mercedes has tweaked all three grades of its retractable-hardtop hottie. But first, let's look at the lineup: For 2009, all models get new shoes and a nose job -- trimmed back a bit, with the wings fleshed out for a more Formula 1 look. Larger mesh in the grille and redesigned foglamps round out the front-end revisions. The back has been cleaned up, too; taillamps are now tinted, exhaust tips are squared off, and the accent lines have been rubbed out. The lower valence has been styled to look like a diffuser. Interior changes are even harder to spot: Redesigners freshened up the instrument cluster and head unit, which has also been updated with Mercedes's next-generation NTG 2.5 electronics system. There's also a new three-spoke steering wheel, which now controls Benz's new Direct Steer system, but we'll get to that in a moment.For individual models, there are few additional changes. The 2009 SLK280 nameplate goes away, replaced by SLK300, even though Mercedes made no change to the 3.0-liter, 228-horsepower V-6. At least its naming convention for its mildest roadster is now consistent. Pricing hasn't been released, but Mercedes informs us it'll stay near 2007 levels.At the extra-spicy-crispy end of the spectrum, the SLK55 AMG remains essentially unchanged, aside from the aforementioned tweaks. Affalterbach's hot-rodster still packs 5.4 liters of V-8 heat, detuned to 355 horsepower and paired with the same paddle-shiftable seven-speed automatic transmission. Additional AMG treatments include sexier aero parts, 18-inch alloys, beefier 325mm brake discs, a sportier flat-bottom steering wheel, and a 200-mph speedometer.The middle-child SLK350 gains the biggest improvements in the form of a significantly modified 3.5-liter engine. Changes include lighter pistons, stronger valves, and conical valve springs, as well as a redesigned intake manifold and higher compression ratio (11.7:1 versus 10.7:1). Camshaft position has also been advanced five degrees on both the intake and exhaust sides. All told, these changes have resulted in gains of 32 horsepower and 7 lb-ft, torque, to 300 and 265 respectively.So with all these changes, is it possible to feel the difference from the driver's seat?No, not really.Don't get me wrong, both the $50,000ish SLK350 and $65,000esque SLK55 AMG are fine-driving vehicles. It's just that, with the top down, who can discern a 32-horsepower improvement? Everything feels fast and sounds good -- particularly when the V-6 sings lustily near its 7200-rpm redline and barks a bit on the seven-speed transmission's throttle-blipped downshifts.Ditto for the 55, though its tone is less tenor and more bellicose baritone; jump off the gas and hear that back-pressure burble. It's positively fearsome when you're top-down and wide open in tunnels and canyons.What you can feel is a decided improvement in the steering feel. Mercedes implemented its new Direct Steer system on the SLK and it's a throwback -- a mechanical solution in the age of complicated electronics and algorithms. The problem Direct Steer attempts to address is the balance between freeway-cruising comfort and canyon-carving sharpness. The heart of this variable ratio system is a steering rack with specially shaped and spaced teeth. A standard pinion gear rides on top, but as it turns the steering ratio can change dramatically and instantly because it's all done mechanically (with hydraulic power assist, of course).Mercedes claims the number of turns from lock to lock has been reduced by 25 percent, and on the road, the SLKs felt noticeably sharper, without that dead spot on-center. More important is that the system is natural and progressive, free of that disconnected, artificial feel many of the steer-by-wire systems have. It's not perfect, but it is a definite improvement.Not a bad a way to sum up the 2009 SLK line.