We’re not fond of most “mid-cycle enhancements,” an oft-used term in the automotive business that has absolutely nothing to do with Cialis or a bicycle’s center of gravity. A mid-cycle enhancement is a car’s midlife face-lift, intended to boost sagging sales until the full redesign can be brought to market. What’s often the case is that these freshenings add little substance, leaving dealers with piles of unsold products and an unfortunate waiting game until the new model debuts.

Such is not the case with Cadillac’s new STS sedan. GM’s luxury unit perfectly made over its new executive sport sedan in two important areas: power and design. We spent the weekend driving the new car and know from both inside and outside the vehicle that the mid-cycle enhancement has done its job. Why? People loved asking us about it and we loved driving it.

The biggest news from a driver’s perspective is that Cadillac has put so much care into its new V-6 that the pricier V-8 seems unnecessary; the two are now just 18 horsepower apart in performance but a quantum leap apart in specification. The new V-6 engine has 302 horsepower, 272 pound-feet, and can hit 0-60 miles per hour in 6.5 seconds, even though full-throttle rips aren’t quite what the car does best. Importantly, it’s GM’s first direct-injection engine, which among other things is more efficient and allows for a higher compression ratio. The engine can also operate in two different modes at start depending on its temperature—a cold-start mode splits ignition timing into two bursts, reducing hydrocarbon emissions and emitting air so clean that it could resuscitate dead baby seals. Cold-start emissions is one of the least ballyhooed areas of the global warming discussion, and one that will surely play an increasing role as the greenies measure our driving time as a function of an ozone hole punch.

What’s interesting to see play out in the real world is that the STS is still a car people love to look at, touch, and ask about. After four years in the market, it’s still strikingly good-looking. The 2008 model’s redesigned front end, which incorporates a new grille reminiscent of the Sixteen show car from 2003 and new fender air extractors, creates a heftier stance and should keep the STS looking fresh until the new model comes to bear in 2010.

The thing we like most about the new STS is that it’s still very easy to drive at low and medium speeds, but now reacts much more quickly to a toe tip on the throttle. Whereas the older V-6 reacted somewhat violently to a full-throttle foot, pulling the wheel and letting out a sharp groan, the new engine gives the driver a significant punch much earlier in the throttle angle. It’s not nearly as loud, which is fine by us. This is an STS, after all, and not an STS-V.

Technologists will appreciate two very innovative features found within some models of the 2008 STS range: a lane-departure system and a blind-spot warning system. The former alerts the driver audibly and visually when he or she has drifted into another lane, but the system rightly quiets itself when it knows the driver is overtaking, either from a throttle tip-in or a turn signal. The blind-spot warning system is a visual-only indicator in the side mirrors that we found very useful and not one bit annoying. Where Volvo and Infiniti brought like systems to market earlier, we’re happy to see GM roll out more useful versions of the technology with fewer user interface problems.

Unfortunately, a mid-cycle enhancement can’t cure everything. The STS’s surprising lack of knee room is still an issue, creating a tight little area for the driver’s knees between the door and center cluster. This sort of problem—which we find frequently in top-end German sedans as well—confuses us given the height and knee-width demographics of the $50,000 sedan buyer.

On the whole, we’re mighty impressed with Cadillac’s new STS. The redesign and additional power should do the car well as it rides off into the product archives over the next three years.