The Zs were part of the factor that my girlfriend and I broke up - we were pushing 9 years on our relationship. She loved the Zs, loved that I did all the work on them, and usually supported me, but I think I just ended up buying too many projects, got in over my head with them, and a lot of what little free time I had (generally worked 70-80 hours a week) was spent messing with the cars. True, they weren't the only reason behind the break-up, but they were definitely a contributing factor.
Ironically enough, this shifted my priorities to where I realize that they're just materialistic things, and even though they're a fun hobby, they really don't bring me true happiness. I've already given one Z to my brother, since he's about to turn 16, and should be down to 1 or 2 soon enough - hell, if I can find enough buyers, I may be out all together.
Don't mean to gloom this thread up, but all I'm saying is to just remember that these are just cars. Even though she may be cool with it, don't let it reign in priority, unless you don't care to keep her, but then you might as well dump her anyways. This is strictly off my experience, but I know a couple of other people that let cars get in the way of their relationship / marriage (again, I'm sure there were other factors, too), and in the end, I really don't think they were happier with the car. Again, could be just me...
I'm sure several other people here have found a nice balance (although I'm sure an offset one) between the two, so I'm sure they'll chime in.