I don't have information on how often they go out, but after reading a few very long threads on the e90post forum it's clearly a problem. The e89 Z4 (2009-2010) 35i have the same twin turbo n54 engine the BMW 335i uses which have been plagued by this problem for some time. BMW extended their HPFP fuel pump warranties to 120,000 miles. Even though BMW indicates they have a new HPFP design it doesn't seem to have helped considering that newer cars are still having the problem.
A number of cars are using direct injection fuel systems these days so one has to wonder why BMW can't seem to get this problem corrected. BMW's new(er) 4.4 L v-8 design 1st used in the x6 50i has direct injection fuel systems and I haven't heard of this problem with their engines. Porsche's newer twin turbo have direct injection and to my knowelage no issue. Somebody else may heard something on it. As you stated the Viper and the Honda S2k haven't exhibited a premature failure scenario, either. BMW - what gives?
Conventional advice for in-tank fuel pumps is try not letting your fuel levels drop below 1/4 tank as the pump needs to be submerged in fuel to stay cool and to last a long time (typ 80-100,000 miles). But if the problem as , the ethanol blend is attacking the pump seals, there really isn't much you can do. If that's the case, it should be a no brainer to fix because ethanol based fuels ain't exactly new tek.
The sound from the pump and the 1/2 vs full tank scenario seems counter intuitive to me. I would have expected the higher fuel levels to dampen the sound rather than enhance it, and the 1/4-1/2 full tanks to be louder due to a sound resonance in the tank, but oh well, you learn something new everyday.
