I'm a huge vehicle fan. I don't have many different types, but I tend to army build them, whether it be a couple of fighters or a tank battalion. I currently have 2 AT-TEs, 2 B-Wings, 4 Spider Droids, etc. My goal for June of next year is to have the following to set up a huge tank battle:
4 AT-APS (UBP)
2 Republic Gunships
4 Hailfire Droids
4 Spider Droids
12 AATs (4 UBP, 4 Clone Wars, 4 UBP)
4 Droid Tri-Fighters
Now, oddly enough, I like the vehicles more than figures. With my goals you might be thinking that I have a huge army, but it's actually pretty small. I don't have near enough Clones from any one source to fill up an AT-TE, let alone four. As for space, that hasn't been an issue so far. Ships hang from the ceiling, AT-TEs are rigged to the wall (vertically) and the small ground vehicles are on shelves. I probably wouldn't open 2 more AT-TEs or all 12 AATs, but rather save them for literally a rainy day in order to make a cool battle display in a more open area of my house.
I don't go quite as crazy with the Imperial/Rebel ships, but that's mostly due to the lack of ground vehicles. If Hasbro made some Rebel tanks, I'd definitly fleet build both those and Imperial Heavy Weapons. I don't currently own any Imperial walkers as I'm waiting for re-done versions of the AT-AT and AT-ST, in addition to waiting for some good TIE Fighters/Interceptors.