My 09 now has 25k miles on it.

My driving is about 30% highway (figure 65mph), 65% suburban (45-50mph) and 5% urban stop and go.

Hand calculated, tracked-to-the-drop mileage over the past 12k miles has been 28.7mpg. I came from a Toyota Highlander Hybrid, and I drive conservatively but I keep up with traffic and I don't super-hypermile. I do pay attention to my speed (I'm not a big speeder), and I do try to make my speed linear in suburban areas with stoplights (i.e. if I'm approaching a red, I will coast for a bit instead of running up to the red and slamming on the brakes). Normal occupancy is me plus about 150# of gear. Onboard computer registers 29.4mpg for same time period. I only buy fuel from busy stations--usually truck stops. Cetane rating is hardly ever provided. X5 is stock with 18" RFTs, and 18" RFT snows in the winter. I have excluded 3 tanks of fuel from these readings: 1 was when I was caught in a blizzard and drove for 9 hours, most of that time not exceeding 10mph, in 18-20" of snow, and the other 2 were tanks that included a substantial amount of trailer towing (one being a ~3500# landscape trailer and the other being a ~2500# enclosed trailer). Mileage for those tanks was obviously worse.

If I'm really aggressive in my driving, the mileage will go down to low 20s (that's based on the car mileage reading, as I've never had a full tank of aggressive driving). Frankly, I don't know how someone could be averaging low 20s unless they were super aggressive or always in stop and go traffic. On the highway, for ~200 mile trips, if I hold 65-68mph here in flat Illinois, it will average over 30mpg. If I bump that up to 75-80, mileage drops to somewhere between 25-27mpg. Off the highway, running the AC affects mileage by somewhere between .5 and 1mpg. Running with my hitch-mounted bike rack full of bikes doesn't affect mileage at all; I suspect the mileage of the X5 could improve on the highway if they cleaned up the rear aero profile.

I'm pretty detail oriented about tracking the mileage, and I realize that this probably seems like one of those boastful online mileage posts (you know, like "My Dodge Cummins gets 45mpg, towing 50,000# uphill in a snowstorm at 80mph"). But it isn't...it is my actual mileage.

When I do drive aggressively, the X5 is one of the best handling, most predictable, and most fun to drive vehicles I've ever had (coming from a long history of SUVs and pickups). Power-on oversteer is fun, predictable, and linear, and she'll hustle through the twisties in a fashion that no 5,000# SUV should be able to. Mine has the stock suspension.

The dirty diesel pumps are a PITA. I've addressed that problem by carrying a box of rubber gloves in the back of my X5, in the perfectly sized cubby on the passenger side rear. Every fuel-up, I grab one glove, slip it on my right hand, and go. I pay with my left hand and pump with my right...and then throw the glove out when done. Cheap, easy, and no diesel smell. I regularly drive around 600miles between fuelups (could go longer, but I start getting nervous). I've never had a problem finding diesel, even in areas I don't normally go...except I do plan ahead so I never have to buy diesel in large cities (e.g. Chicago). I do occasionally run into an oversized fuel station, but have never found a station selling diesel that doesn't have at least one properly sized pump nozzle. In the past year, diesel has been more expensive than gas in Chicago's western suburbs for about one month, total. I fueled up last evening in DeKalb, and paid $3.90 for diesel; regular unleaded was $3.99. I can't complain about that.

I have more fun with my X5 than I have any right to, and it performs better than I have any right to expect. I see absolutely no downside in the comparison against a 6 cylinder gas X5. And true--you can never have too much power--but my X5 has never left me wanting for power in any situation. Given the choice, I'd buy another X5d in a heartbeat. I have previously had diesels, and enjoy the way my X5 drives.

I'm not sure what the current gas/diesel cost differential is or if that's a factor in your decision. When I bought, I was buying a $60-70k car, so a minor difference in fuel price was not a decisive factor. However, I do a lot of driving, and if I can have the same or a better driving experience and get substantially better mileage, I'll take that option. For the sake of comparison, assume a 6 cylinder gas would get 20mpg, and a diesel would get 27. Even though diesel is cheaper than gas here, we'll use $4/gallon as the price for both fuels. That's $.20/mile for gas, and $.14/mile for diesel. Maintenance costs are the same, with BMW's included maintenance. Every 10,000 miles, that's an extra $600 for gas. Not a dealbreaker either way, but still, a not insignificant sum.