Where you drive has a lot to do with your impressions of the BMW 5 Series sedans. If there's an autobahn outside your front door, it's only natural to look at speed potential. A narrow, winding road built into the daily commute focuses attention on suspension design, while a stretch of speed-limited smooth freeway brings considerations of silence, comfort and, perhaps, gas mileage to the fore. Logic suggests that any car capable of standout performance in any one of these environments will be compromised to a notable degree in the others. Fuel efficiency and velocities high enough to allow use of the fast lane on a German superhighway don't often mix; neither do responsive handling and cruising comfort. In most cases, anyway. But BMW's 5 Series lineup is something special. We've had the opportunity to spend time in these sedans in all of the situations listed above, from foot-to-the-floor hurry-up mode to back-road barnstorming to gentle commuting. The conclusion: Competence is always in evidence, and comprom