I've had my batteries die midflight and whilst it's quite obvious that they have died they certainly appear no worse with the ANR turned off than they were pre mod. I always carry a spare 9V in the headset bag and it can be swapped over in 15 seconds flat anyway. The book says 15 hours per battery set, I'd estimate mine would last well over 25-30.
I would suggest spending the money and having them installed professionally, looks wise they look no different except you do have to drill a hole in the cup, that runs out the cord to the battery pack, I just used cable ties to tie the ANR cord to the mic plugs cord and put the power supply in the sidepocket or on the floor, just keeps it a bit tidier than having another cable lying around. Definately suggest them to anyone, i've let a few people borrow them and they have all been quite impressed. You can't use the comfort covers with them though because the ANR works best with a good seal to keep the noise out.