Having given this thread a good read, and some thought, my 2 penneth worth is as follows:
Carrying that weight around all day - don't see an issue. Used to walk around with a 5D2 with BG + 70-300 L on for 8-10 hours without an issue. Some times even had a 580 EXII attached

A lot of people were unable to handle that combination due to weight, yet i never once felt it to be heavy. So don't let weight put you off.
As said in a previous reply - when ever you look at the pics of birds that jump out, then check the kit used - you always see something in the region of 500mm being used, and just as often with a 1Dxx body - and these are just hobbyists.
Price - yes, it is a lot. But it's your money - and if your GF is just as interested in you getting the lens - then get it. This is your hobby, your interest and you have to work to live, not live to work. Photographing birds generally seems to involve a beast of a lens, and as much as i loved the 70-300 L, i can happily accept that it struggled with reach on occasions - but i don't shoot birds, nor do i do much shooting that requires 300mm let alone 500mm!
Theft, accident, fire and all those other things - you live in Austria not the 'States'. Sorry guys, but things are just a little bit more relaxed in many parts of Europe - especially Austria (along with a few others).
Travelling around on a bike - surely the real question here is 'why can't you get a bike panier to carry photographic kit safely?' ??? Again, in Europe we don't all have pick ups or 4x4's as a standard mode of motorised transport. A bike is not only quieter, but can often get you places where a car won't! You can also park it up very quickly for the 'chance shot' you see when travelling at 10-15mph as opposed to 30-70mph! 8)
So, with all the above in mind, my only caveat would be to rent it if possible, if not - then heck, it's your money and your life...