...of a saying: "Helicopters can't fly. They're just so ugly, the Earth repels them".
Ugly they are (like all post-2001 BMWs, except, maybe, E92). It is difficult, though, to turn down the appeal of the powerplant previously available only in a grownup category.
Here's a comparison - E39 540i 2003 vs. E81 135i 2008:
Power: 290 bhp vs 300 bhp
Torque: 325 lb-ft vs 300 lb-ft
Weight: 3757 lb vs 3438 lb
Power to weight: 0.0771 vs 0.0926
Torque to weight: 0.0865 vs 0.0926
One interesting fact: specified weight of 135i is just 0.96 of the weight of 335i, and 0.87 of the weight of 550i for the same model year. It's not light, no matter how they try to convince you it is, it's just small.
For comparison, 135i has a weight that is 1.73 of Lotus Elise (totally different ballgame), and 1.21 of Acura RSX Type-S (arguably comparable level of fun, performance and luxury, but absolutely different demographics).
Likewise, I don't want to bother with calculating the prices, but something tells me that you might just get yourself a 3 series or 5 series for almost the same cost, percentage wise... Just don't get yourself blinded with the absolute difference - think "relative" and "cost efficiency".