To sum it up, you don't really get them for "free". Yes, you can own what ever you buy, but everytime you want to use one, you have to call in to have it delivered to you - which costs 1k each time. That will eventually add up. Especially if you don't have that much money.
Since most of those spawn in-game, I seriously just think the tank is worth it. Then again if someone has a sticky bomb or what ever you're pretty much toast..
I've just bought the seadoo only. Don't think I'll be buying any special vehicle unless R* changes how you have to call and pay 1k.