I've owned a Zulu 1 for some years and recently performed a rather extensive test with loaned headsets: A20, Zulu 2, Sennheiser S1. Here's my view:
Zulu:
+ good active attenuation
+ good bluetooth support for music and telephony
+ good sound quality for stereo music
+ great support with excellent turnaround time for repairs (Germany rep)
- terrible passive attenuation (drained battery)
- poor reliability and quality, PCB was replaced twice and several plastic parts broke, the original volume controls are subject to wear out and corrosion. A lot of cheap China made stuff but apparently that is better with the Zulu 2.
- uncomfortable with certain type of glasses
Sennheiser:
+ good active attenuation
+ good passive attenuation
+ great bluetooth
+ great comfort level with glasses
- huge and heavy
Bose:
+ great active attenuation (the best I found)
+ good comfort level, also with glasses
+ very compact and lightweight
- bad passive attenuation
- ugly "pubic hair" pad
- completely stupid bluetooth support, only does phones which is almost useless unless on the ground, no support for pairing music players
After a lot of tinkering, I went for the A20 without bluetooth. It was a tough call but eventually the Sennheiser's weight made me go for the Bose.
Something interesting that the Bose reps told me: their attenuation circuit is analog whereas the Sennheiser use a digital circuit, i.e. they digitize the voice signals, process them and convert them to analog again and according to Bose this induces a delay which makes the processing less effective than their analog circuit. I am not expert enough to judge this argument.